South-East Asian Fortified Stone Walls : Angkor Thom ( Cambodia ) , Ho Citadel ( Vietnam ) and Ratu Boko ( Indonesia )

This article aims to analyze three significant examples of defensive walls from SouthEast Asia made of solid stone blocks (both rock as well as stone-like laterite) and provided with different but equivalent functions –a fortified imperial capital-city (Angkor Thom, in Cambodia), a fortified royal citadel (Ho Citadel, in the North of Vietnam) and a royal palace with a partly fortified appearance (Ratu Boko, in Java Island, Indonesia)–, focusing on their constructive and technical characteristics and establishing parallels between them and their closest counterparts, from China and India. We will see how their design and structure can be closely related to the fortifications of neighbouring empires, as places of origin of their strong cultural influences and, at the same time, we will try to identify the local particularities. We will pay special attention to the form of the fortified enceintes, considering the long tradition of the quadrangular plan in the walls of royal capitals, inspired in the ideal model of Chinese and Indian cities. Our research also make us think that the walls of Ratu Boko, despite their functions as symbolic limits or for retaining the soil, could also have had a defensive purpose, no matter if secondary, or at least they could be used to provide protection to the complex in case of external menace.


INTRODUCTION
This article is the result of research conducted during a series of study visits to three important archaeological sites in Southeast Asia, focused on the analysis of stone walls -made from both rock as well as stone-like laterite-erected with a defensive function 1 .The most significant examples of the type in this geographical area undoubtedly can be located in Cambodia and Vietnam: Angkor Thom, in Siem Reap province (visited in 2009), and Ho Citadel, in Thanh Hóa province (personal collaboration with the local UNESCO staff in 2013).Furthermore, I also considered an outstanding case from Indonesia which served to establish interesting comparative links: Ratu Boko, in Yogyakarta (visited in 2014).
The two first monumental compounds were designated World Heritage sites by the United Nations Educational, Scientific and Cultural Organization (dates of inscription: 1992 and 2011, respectively), while the last one was included in a Tentative List (1995), due to their uniqueness and great historical-architectural value.In addition, each of them constitutes a different kind of civil structure provided with outer stone walls: a fortified imperial capital-city (Angkor Thom), a fortified royal citadel (Ho Citadel) and a royal palace with a partly fortified appearance (Ratu Boko).These were the main reasons for their choice as objects of study in my research.
Unlike the regions of East Asia (China 2 , Japan 3 , Korea 4 ) and South Asia (India 5 ), with abundant and ancient traditions of building stone fortifications (including city walls, military fortresses, castles and frontier long walls), Southeast Asia (mainland and insular) is very scarce in them.Also noticeable is the inexistence of specialized publications, neither strictly academic nor merely informative, devoted to the particular subject.Therefore, while examining attentively each one of these cases, I am going to focus on the constructive and technical characteristics of the stone perimetral walls and to try to establish parallels between them and their closest counterparts, from China and India.

ANGKOR THOM (CAMBODIA)
This is the most imposing stone fortification in the whole Southeast Asia, considering the monumental appearance of its gates, the uniqueness of its artistic decoration as well its remarkably long perimeter and overall huge size.
It is located in Siem Reap province, in the northwest of modern Cambodia, only about 20 km from Tonle Sap ("Great Lake"), the largest freshwater resource in the whole Indochinese Peninsula.The city was built on the west bank of the Siem Reap River, a tributary of Tonle Sap, in a plain terrain seasonally affected by floods, naturally surrounded by the tropical jungle.
The urban enceinte which can be visited nowadays belongs to the third capital city which was successively constructed in Angkor, all of them with vast regular quadrangular layouts and surrounded by wide moats filled with water.The first one, called Yasodharapura, was established by King Yasovarman I (889-910) and consisted of a square of 4 km x 4 km (1600 ha) centred on the Phnom Bakheng temple.The second one, also with a square perimeter of 4 km x 4 km, was erected by King Udayadityavarman II (1050-1066) to the north and had the Baphuon temple in the middle.After the sack of Angkor by the Cham (1177), King Jayavarman VII (1181-1218), the most powerful Khmer monarch of all time, built a new capital for his empire, Angkor Thom, which kept this role until it was abandoned in 1432 6 ; the magnificent temple of Bayon was placed in the centre, in the intersection of the two main roads, and the new city walls comprised part of the areas from the two previous capitals.
In its time Angkor Thom 7 was the biggest city of Kambujadesa or Khmer Empire (802-1431), with an extension of 9 square kilometres (900 ha) and a remarkable population of nearly one million of inhabitants, including the citizens who lived inside and outside the walls 8 .Today it is probably the most important archaeological site of the whole Southeast Asia.Drawing an square of almost 3 km per side (3045 x 2927 m), its defences consisted of a solid wall 9 of almost 8 meters of height, bordered externally by a moat (100 metres wide and 6 m deep provided with drainage tunnels) and reinforced on the interior by an earthen rampart (15 m wide and 45 degrees of inclination) that forms a broad road around the interior of the perimeter.Both the wall and the embankment are pierced by small gutters for draining the water from the city towards the moats 10 .
Angkor Thom has five monumental tower-like gates or gopuras (23 m high), one in each side of the enceinte and a fifth one in the east corner 11 .With a cross-shaped plan, all of them have a single gateway (3'5 m wide, 7 m high and 16 m long) covered with a corbel or false vault, crowned with four distinctive gigantic stone faces representing the guardian bodhisattva Avalokitesvara which look towards the four cardinal directions, decorated with mythical three-headed elephants flanking the entrances, and preceded by an avenue of gods (devas), demons (asuras) and seven-headed monstrous serpents (nagas) lining the stone bridge (16 m long) across the moat.
The curtain walls were made mainly of hard rectangular laterite blocks laid in regular rows without joint mortar and buttressed by earth, with a parapet with carved corbel arches on the top and a decorative base in the bottom, both of them separated by mouldings.The type of laterite found in Angkor is limonite, rich in nickel.This reddish natural material was broadly used in South Asia, from India until the Indochinese Peninsula, throughout history.Although laterite is not technically a stone, it resembles a rock, given its aspect, composition and solidness 12 .That is the reason why I considered the analysis of the enceinte of Angkor Thom together with other main stone fortifications from South East Asia, discarding however those made of bricks.In addition to laterite, sandstone, a soft sedimentary rock obtained by Khmer builders from the Kulen mountains, was used as a secondary material due to its highest quality and cost in some important parts of the city wall that protected Angkor Thom, like the carved gates and the preceding causeways, as well as the corbel vault which crowns longitudinally the laterite walls.

HO CITADEL (VIETNAM)
This is probably the best example of a stone fortification in the whole Southeast Asia, in relation to its massive and impregnable appearance, its magnificent regular structure as well as its notable conservation status.
It is found in Vinh Loc district, Thanh Hóa province, in the north of Vietnam 13 , about 250 km from the modern Communist Republic of China.The chosen location was a fertile plain terrain of rice fields comprised between Ma River (West) and Buoi River (East), and Mount Tuong Son (North) and Mount Don Son (South), a place of strategic importance.It was built in 1397, in only three months, by Ho Quy Ly King, founder of the ephemeral Ho dynasty, to house the new capital of the country 14 .In 1407 the Chinese Ming Empire conquered the kingdom of Dai Viet and the seat of the power was again transferred back to Thang Long (ancient Hanoi) 15 .However, the citadel kept its role as a military fortification and remained also as the political, economic and cultural centre of the region from the 16 th to the 18 th century 16 .Although the stone-faced ramparts of the fortress were partly destroyed during the turbulent period of the Tay Son rebellions (1778-1802), all of its perimeter is well preserved nowadays to a considerable height.
An impressive inner stone wall marked the limits of the royal citadel, the centre of the city.Perfectly designed and firmly constructed, it has a square layout of 877 m (north and south sides) x 880 m (east and west sides), that is 77 ha, devoid of towers.There is a great gate at the centre of each side: the North, East and West gates had only one single entrance, while the principal one (at the South, measuring 15 m wide) had three gateways, all of them precisely covered by semicircular arches or barrel vaults composed by wedge-shaped voussoirs, and with the central arch slightly larger than the lateral arches.Only the South and North gates have kept their upper terraces, on the top of which large round holes bear witness to the former existence of wooden pavilions.Those four gates were connected by the main axis of the citadel, two inner roads (4.85 m wide) paved with flat stones which crossed in the centre and which have survived until the present time.
Technically, the walls consist of thick earth ramparts faced externally and internally by layers of ashlars, large limestone blocks meticulously shaped and arranged in perfect regular rows.Large size stones which reach the weight of 26.7 tons were placed at foot to consolidate the base and smaller rectangular stones placed on them 17 .It has been estimated that more than 100,000 cubic metres of earth and 20,000 of stone hewn from local quarries were used in its construction.The top of the wall was crowned with brick defensive structures, which are not visible anymore.The gates and the corners of the curtain walls have a slightly tapering form, given its inclination towards the interior.
The walls of the citadel were surrounded on all the sides by moats originally filled with water with an average width of 50 m; although nowadays they are mostly covered by earth, its vestiges are still clearly identifiable in every direction.An outer wall (La Thanh) protected the rest of the buildings and residences of city.With a total perimeter of about 10 km (enclosing thus a surface of 600-700 ha), it had an irregular layout, adapted to the local topography, the surrounding rivers and hills.For the most part the natural setting was used with defensive purposes, while other parts were reinforced with earth ramparts, ditches and bamboo brushwood fences.The best preserved remains of the rampart can be seen in the fields of Vinh Phuc (to the southeast of the inner citadel) and of Vinh Long (to the northeast), which measure about 4 km long in total 18 .

RATU BOKO (INDONESIA)
This is a unique case of a stone fortification in Southeast Asia.Unlike the city of Angkor Thom and the Ho Citadel, the whole archaeological area of Ratu Boko was perfectly not closed with a sole continuous perimetral enceinte, its fortifications don't constitute a structural unity and the walls don't have a so impregnable appearance as those from Angkor and Ho Citadel.Another difference to be highlighted is the location, an easy-todefend hill position with visual control over the surrounding territories, instead of a vulnerable plain which would have required stronger artificial defences.Finally, although its stone walls served for defensive purposes, besides showing power and prestige, some of them also had an additional practical function such as enclosing a building or retaining the soil of terraces.
Kraton Ratu Boko, which in Javanese language means "palace of King Boko" 19 , was one of the royal residences of ancient Mataram State as well a centre of religious importance, commenced construction in 760-780, in the time of King Panangkaran 20 , later transformed by King Rakai Walaing Pu Kumbhayoni 21 (856-863) and continued to be inhabited up to the 14 th -15 th century.Mataram was a Hindu-Buddhist kingdom (8 th and 9 th centuries) based in Central and East Java 22 .The capital was placed by its founder, King Sanjaya, in the former region of Mataram (modern Yogyakarta), which comprised the fertile volcanic plains of Kedu (Borobudur) and Kewu (also known by the name of Prambanan).Ratu Boko was erected on a small plateau of 16 hectares, at the top of a hill of 160-200 meters of altitude above the sea level, about three kilometres to the south of Prambanan Temple, in the major archaeological area of the whole Indonesia 23 .
The monumental site of Ratu Boko is formed by three compounds: one in the north-west, another in the south-east, both of them well known and studied, while the north-eastern one, the smaller, has not been fully excavated yet.Those compounds are formed by successive terraces on different levels, divided by several courtyards bounded by retaining walls or enclosures.Since most of the structures have been built of wood and other organic material, only their stone bases and walls remain nowadays.The site comprises several types of buildings: audience hall (pendopo), princess palace or women's quarter (kaputren), reception pavilion (paseban), royal baths, temples, shrines, crematorium and meditation caves carved into the rock 24 .
Stone-faced ramparts, freestanding stone walls and dry moats (not filled with water) are the fortified elements of the palace complex which, albeit present everywhere, are concentrated mainly in the north-western part of the plateau, facing the sole access path 25 .The second and third terraces are sustained by walls of about 170 m long (N-S), the second one slightly higher (5 m) than the third one (4 m); between those two walls there is an intermediate fence (3 m high) running parallel to them and pierced by the main gate, which appear again in the south side of the third terrace 26 .The E-W retaining wall of the third terrace measures about 130 m.In the base of the second terrace a narrow moat was excavated, while the third terrace has in both of its sides a stone paved flat-bottom ditch or open duct (about 1 m wide), as well as the remnants of a front wall or fence, today only preserved in its original height (3 m) in one part, similar to a detached military outwork.
The south-east area has two terraced retaining walls of 150 and 130 m respectively (N-S), the second one of them provided with the same kind of ditch and front wall or fence.In that compound can be found the outstanding stone enclosure of a building identified as the Audience Hall (pendopo).It has a rectangular layout of 40.8 m (N-S) x 33.9 m (E-W), pierced by three simple doors (north, west and south), and the walls, measuring 3.45 m high, are the finely shaped and crowned with a row of decorative pinnacles.Near there, in a lower terrace, the bathing complex also has a massive rectangular stone enclosure of around 50 m (E-W) x 60 m (N-S), with a gate in the middle of each side.In the north-eastern compound, atop the hill, there are remains of an enclosure wall.
Two kinds of stone were used in the construction of the fortifications: the retaining walls of the terraces, the front freestanding walls or outworks and the enclosure of the baths are made of white limestone, a soft sedimentary rock, while the main gateway and the walled enclosure of the Audience Hall were made of andesite, a hard igneous rock.Thus, the best quality material was reserved for the most important structures of the palace site.As for the masonry technique, although in the Audience Hall the stone was cut in rectangular blocks (ashlars) arranged in regular rows, in other places the blocks didn't have a so regular a shape nor were arranged in such a perfect position.It is possible to observe with the naked eye the inner composition of the walls of baths: these are formed by two external facings of ashlars and a filling of irregular stones mixed with mortar.By contrast, the outer walls that surround the ditches of the terraces are composed of three layers of ashlars, two externals and one internal.The retaining walls of the terraces have a slope, while the walls of the Audience Hall and the bathing complex as well as the freestanding walls erected in front of the stone-faced ramparts have a vertical position.
The main gate, located at the westernmost past of the site, consists of two successive gateways (gopura), located at two levels in different terraces and connected by stairs: the first gateway has three rectangular doors -that is, covered with straight architraves instead of arches or vaults-, while the second one, bigger, has five doors 27 .This is at the same time the most fortified point of the palace complex and also the most monumental one, due to the double necessity of protecting a week point (access) and also of impressing the visitor with a magnificent architectural design.We can find the remains of inner gates and stairs at the E-W side of the third north terrace, as well as in the N-S side of the south terrace 28 .

COMPARISONS AND CONCLUSIONS
Although these three cases might appear to be isolated examples in their regional context, in fact their design, structure and technical characteristics can be closely related to the fortifications of neighbouring empires (China and India), as places of origin of their strong cultural influences.On the one hand, Angkor Thom and Ratu Boko, like any other architectural production of the Hindu-Buddhist states which flourished during the first millennium AD and the half of the second one in Southeast Asia, have been clearly influenced by the old Indian tradition.On the other hand, the construction of Ho Citadel followed the typical precepts of the millenarian Chinese tradition.In order to better understand this fact, we should keep in mind that the Khmer Empire (Cambodia) as well as the Medang Kingdom (Indonesia), formed part of the Indian cultural sphere or Indosphere 29 , whereas the Dai Viet Kingdom (Vietnam) was integrated into the Chinese cultural sphere or Sinosphere 30 .
Let us examine in the first place the particular case of Ho Citadel.Culturally and historically we should distinguish between the Viet and the Champa territory (in the north and in the south of modern Vietnam), which were respectively influenced by the Chinese and Indian civilizations.It was not until the beginning of the 19 th century when the Vietnamese ended to conquer the Cham land.Hence it's not surprising to find the capital of the ephemeral Ho Dynasty in the north, as well as Hanoi, the traditional Viet capital, both of them planned and fortified according to the Chinese principles of geomancy, urbanism and military architecture.Obviously 1400 years of direct political Chinese domination left a strong mark on the country, starting from the design of capital cities with their regular defensive walls.
According to the ancient Chinese texts referring to architecture and urbanism 31 , the ideal city consisted of a square, a theory which may have derived from the belief that the heavens were round and the earth was square.And just as the earth has four cardinal points, also the city should have gates in those directions, connected by main roads which would serve as the central axis for arranging the inner space according to a grid pattern, symbol of order.Although this shape was the preferred norm in the flat plains of northern China, the topographical conditions sometimes determined the adoption of irregular perimeters in the rougher territory of the south 32 .The selection of the site was determined by the search of a favourable geographical position; thus, feng shui recommended to locate cities on level land with a river in the front and a mountain behind.In the capitals, the imperial palace should be placed in a pre-eminent position, which varied from the centre to the north depending on the historical period.The wall symbolized the sovereign's reign on the earth.And the importance of the surrounding enceinte is revealed by the fact that a same word was used in Chinese to design a city and a city wall (cheng) 33 .
We should take now a quick glance at the general characteristics of the Chinese urban fortifications in order to establish some morphological comparisons 34 .The main similarities between them and the defensive system of the Ho Citadel are: quadrangular plan, thick walls of massive appearance, wide moats preceding the curtain walls 35 , presence of a distinctive inner city or citadel and an outer city provided both of them with perimeter walls 36 , and monumental gates in all the cardinal points often covered with barrel vaults and provided with wooden upper structures (gate towers).However, amongst the differences, we can notice at the Ho Citadel the absence of barbicans or defensive courtyards in the gates, the non-existence of projecting square towers arranged at regular distance, archaeologist couldn't find any trace of the typical Chinese "sheep-horse walls" (yang-ma cheng) or low preceding walls (proteichisma) erected between the main walls and the moat, and finally, although the walls are composted by a core of rammed earth the facing is not made of bricks -as was frequent in the whole China-but of stones.
The closest parallel, in the time and space, of a Chinese capital city with ashlars in their fortifications can be found in Nanjing, imperial seat of the Ming Dynasty between the years 1368 and 1421, coinciding with the foundation and Chinese conquest of the Ho Citadel.Nevertheless, the curtain walls of Nanjing 37 (1368-1389) were not only faced with stone but still with brick as the main material.Moreover, their stone blocks were not all rectangular nor as huge as those from the Ho Citadel.
As for the rest of royal citadels of the Viet kingdom before the 19 th century, all of them were protected with brick and/or earth enceintes -making thus the limestone walls of the Ho Citadel more exceptional in its context-and had a quadrangular layout.This is even the case of Co Loa Citadel (Hanoi) in the period previous to the first Chinese domination of the country, with a rectangular earthwork and brick citadel (1.65 km of perimeter) dated from 3 rd century BC surrounded by two outer irregular ramparts provided with towers (6.5 and 8 km) and three moats filled with water from Hong River 38 .The Thang Long Citadel (Hanoi), capital city between the 11 th and 18 th centuries, was constructed also brick in 1029 on the remains of the previous Chinese fortress dating from the 7 th century, with a rectangular perimeter, four gates oriented towards the cardinals points and a Chinese imperial layout; in addition, an inner and an outer irregular enclosures of earth protected the city 39 .
There is another case of stone-like fortification in Vietnam, further to the South.However, in contrast to the Ho Citadel, the rectangular earth and laterite walls (1400 x 1100 m) of the Cha Ban or Vijaya citadel (Canh Tien, Binh Dinh), capital of an Indianised Champa kingdom between the 11 th century and the Viet invasion in 1471, reminds the Hindu-Buddhist style of stone or laterite fortifications 40 .Moreover, unlike other Champa centres which used bricks as a primary material of construction, the abundant well-cut stone and laterite blocks from Vijaya reveal a major influence from the neighbouring Khmer Empire, especially from Angkor Thom 41 .
The closest parallel to the city walls of Angkor Thom, regarding to the geographical proximity, shape, design and chronology of the defensive perimeter, can be found in Phimai (ancient Vimayapura, nowadays located in modern Thailand), an important Khmer town.Its urban fortifications draw a rectangle (1033 x 665 m) constructed with laterite blocks in the time of King Jayavarman VII (1181-1218), replacing the previous fortifications from the 11 th century 42 .Quadrangular capital cities which followed the principles of Indian urbanism and cosmology were frequent in the Indochinese Peninsula: Sukhothai (Thailand, founded in 1238), with a complex rectangular defensive system (1800 x 1360 m) composed by a wall of bricks and laterite, two embankments and three moats; Chiang Mai (Thailand, fortified in 1336-1355), with a brick city wall and a moat of 1750 m x 1750 m; Amarapura and Mandalay (founded in 1782 and 1857 in Burma, present Myanmar), both of them with a square-shaped brick city wall preceded by a moat (1000 m and 2000 m per side, respectively) 43 .However, in all these cases -as well as in others not cited here-, brick was the material broadly used, not laterite like in the Khmer cases of Angkor Thom or Phimai.
According to Indian technical treatises on architecture and urbanism (vastu-shastras) compiled in the Antiquity and influential until the arrival of Muslims, a military camp, fort, village or town, either circular or square in plan, should be encircled by ramparts and moats and were supposed to have two major axial roads aligned with the four cardinal points.A capital city not only served as the political centre of the state and the seat of the monarchy, but also it was a representation of the whole Indian universe, like a microcosm.The main temple was a replica in the Earth of the mythological Mount Meru, and the whole city was erected around it.Hence, the main temple was installed at the central crossing of the two axial roads, while the king palace to its north.And within the walls, akin to a sacrificial enclosure, there was a ring road corresponding to the path of the auspiciousness around a sacred site.However, those rules were more theoretical than practical, although Angkor Thom is a perfect example of this conventional foundational plan of a Hindu capital city 44 .
Archaeology reveals that the quadrangular form of city plan was frequently used in Burma, territory which constitutes the entrance gate to the Indochinese Peninsula from India, since the first millennium of the Christian era until the British conquest in the 19th century) 45 , a fact that helps to explain the spread of this urban tradition into neighbouring countries like Thailand or Cambodia together with the spread of the Hindu-Buddhist culture and architecture in particular.Nevertheless, it seems that more efforts were made to apply the theoretical principles of the ideal city according to the ancient and medieval Hindu urbanism and cosmology in the South-East Asia, that is in the Indian influenced kingdoms, rather than in the Hindustan Peninsula were they were originated.
In India the regular designs of the plans which resemble mandalas are found in religious enceintes instead of fortified towns.
A notable exception are the meticulously planned fortifications of the square cities of Jaugada and Kalinganagara in Shishupalgarh (Orissa), dated from 3 rd and 2 nd centuries BC 46 .The defences of Jaugada consisted of a wide earthen rampart (900 x 1050 m, 45 m wide and 7 m high) with two protruding gates per side and a moat 47 .The defences of Kalinganagara consisted of a laterite wall facing a rampart oriented along the cardinal directions (1200 x 1160 m, 4.7 km of perimeter, 3 m wide wall, 36 m wide and 16 m high rampart), with bastions in the corners, two protruding main gates per side and a moat fed by a nearby stream.In comparison with the gates of Angkor Thom (both of them wide enough to allow the access of chariots or elephants), those of Kalinganagara are more efficient in terms of military defensive technique, thanks to its complex structure provided with barbicans, opened at the top so that invaders could be bombarded with arrows from all directions 48 , while in Angkor Thom decorative giant faces were preferred.
We should take now a rapid glance at the general characteristics of Hindu fortifications in order to establish some morphological comparisons 49 .The main similarities between the strongholds of the Indian subcontinent (during the first millennium and until the 14 th century 50 ) and those from the South-East Asian Indian influenced countries are: solid curtain walls composed of high and thick earthen embankments faced with hard material (stone, laterite or bricks) laid without mortar and preceded by wide and deep moats, and the use of corbel arches or monolithic lintels to cover relatively simple gateways 51 .Several differences can be noted, notwithstanding: prevalence of fortifications in India with and irregular layouts over regular quadrangular plans, preference for high positions for defence (rocky hills whenever possible) over plain terrains, major use of stone blocks over bricks as the priority material of construction, more extensive use of towers and of defensive structures designed to resist a massive military siege or the attack of sophisticated war engines 52 .We can add one more difference: curtain walls tended to be more massive and thicker in India than in Angkor Thom.
As for the last of the three stone walls examined in this article, Ratu Boko, it is a particular case which requires more careful attention given its controversial nature.Indeed, the function of this site had been widely debated by the researchers, both by field archaeologists and analyst scholars, and in some occasions the fortified character of the structures was even put in doubt.Nevertheless, in my opinion, the different stone walls not only had a function of retaining the soil of the terraces in the case of the sloping ramparts, or a symbolic function (prestige, exaltation of power and/or separation from the rest of the world) in the case of the free standing enclosures, but they also had a defensive purpose, no matter if secondary, or at least they can be used to provide protection to the complex in case of external menace.We should keep in mind that, unlike Angkor Thom and the Ho Citadel, which were founded on plain terrain vulnerable to the attack of big armies, the elevated position of Ratu Boko and the presence of escarpments in the hill made the construction of massive impregnable fortification works unnecessary.
On the other hand, it is very probable that the centre evolved from a pre-eminently religious one towards a political one, always maintaining a close link between the secular authority and the state cult, making thus impossible to separate them in the proposed interpretations.Although it seems that Ratu Boko was originally (760-780) designed as a Buddhist monastery promoted by royal patronage, about one hundred years later (856-863) the site was transformed into a royal palace 53 by a Hindu ruler, devoted to the cult of Shiva.
While some authors considered the site as a royal palace garden 54 , others had underlined mainly the influence of the Sri Lankan Buddhist monastery Abhayagiri Vihara 55 (Anuradhapura), some centuries older.The comparisons have been established according to these similarities: name, location on a hill, presence of meditation halls or caves for ascetics 56 , and alleged architectural parallels 57 .Anyway, I prefer to highlight these evidences: on the one hand, both sites were constructed thanks to the patronage of local rulers and served both as religious centres and royal capitals provided with recreational areas for the kings 58 ; on the other hand, Ratu Boko experienced an evolution over the time and had multiple functions, including a fortified residence of the civil authority, which particularly attracts my attention.
Although constructed later, between the 12 th and the 13 th centuries, I would mention here another parallel of different nature: the imperial palace of Angkor Thom.Similar to Ratu Boko but at higher scale, it also contains a fortified perimeter (moat and rectangular laterite walls of 246 x 585 m), monumental gates with multiple entrances (gopura), pools and ponds, religious structures (a temple from the end of the 10 th century), royal terraces (stone foundations for pavilions) and a great number of buildings including residences for the king, for his wives and concubines, guards and soldiers, all of them erected with perishable materials which have been long vanished 59 .
Kraton (or keraton) is a type of native Indonesian traditional building which historically had functioned as the residence of a royal family 60 .A fundamental component of a kraton is the pendopo, a quadrangular wooden pavilion structure built on columns, used to shelter the institutions of ancient Javanese kingdoms, such as law courts, clergy, palaces, and for public appearances of the king and his ministers 61 .The elevated platforms, the bases for pillars (umpak), the design of the plan and other archaeological indices reveal clearly the former existence of this kind of pavilions in Ratu Boko.Given the perishability of the organic materials, the remains of the ancient pendopos are often limited to the lower stone or brick elements, like in the case of the original royal pavilions of Trowulan, the 14 th century capital city of the Majapahit Empire.The first timber structures preserved from the 15 th (kraton Kasepuhan in Cirebon), 16 th and 17 th centuries suggest that the design of pendopos had not changed much for over a millennia, keeping the same structural elements since the 8 th century Ratu Boko remains and the 9 th century representations from the Borobudur reliefs 62 .
The moat and the stone walls of the 16 th century Kraton Kotagede 63 (Yogyakarta) remind us that the Javanese palaces were protected by defensive works, the same as the palaces of other neighbouring Indian influenced kingdoms like Angkor Thom; therefore, Ratu Boko is not an exceptional case, but the oldest remains in its category.Actually, all palaces since the Medang Kingdom (8 th -9 th c.) were a collection of pendopo style pavilions and halls surrounded by walls.Considering that the Indonesian urban centres did not fully develop until the 13 th century 64 , in this period the cities from Java were not walled like those from Europe, Middle East, China or India.Hence, the only well-guarded and protected constructions were the king's palace and temple compound 65 .
As for the most direct influence of the architecture of Ratu Boko, this can be appreciated in nearby constructions from Yogyakarta area just a few decades later, although they are of a purely religious nature.This is the case of the perimeter walls of the Hindu Prambanan temple complex, especially the inner enceinte, and the upper terrace of the much smaller Hindu Barong sanctuary, with sloped retaining walls and a double successive gate (gopura) 66 , both of them dated in the middle of the 9 th century.The central courtyard of Prambanan is the best example of large walls built the double facing technique, very popular in that time in Java, similar to free standings enclosure walls of Ratu Boko: perfectly adjusted volcanic stones were used for the façades, while the inner part was made of cheaper materials, crudely hewn limestone blocs and a mixture of lime and mud 67 .
Finally, although the size is not as colossal or impressive as other fortresses from Mainland Southeast Asia, we should remark that the archaeological and historical value of the defences of Ratu Boko is due to the exceptionality of solid non-perishable material in that period for protection purposes (no matter if primary or secondary): as Chinese written sources from the 9 th century state when describing the country and its inhabitants, in that time the Javanese people made fortifications of wood 68 .Such an early use of stone in walls, embankments and ditches reveals the important role of the site for the society and political authorities, which deserved to be considered in this study.