Perhaps the earliest temple-mountain to survive at Angkor is the small Prei Prasat, in the north-west corner of the irregular grid of canals and roads that forms Angkor. It was built on an island site, without encircling walls, surrounded by a backwater of the canal system. Prei Prasat was constructed during the reign of the great Jayavarman II, from A.D. 802 to 850, and already the familiar steep step-pyramid shape can be seen. The building materials were chiefly brick and stucco plaster, with detailing in laterite-a soft stone that hardens after exposure to the air. Architecture and sculpture of this period has been dubbed the style of the Kulens by art historians. It digested influences from the Khmers' Chenla kingdom and from the Java of Jayavarman's exile. The architectural forms, here in Angkor and other Khmer cities, such as Sambor, are vigorous and carved with lively and naturalistic decoration of foliage and people. By the end of the long reign, some of this vivacity of detail had turned into stylized conventions, especially in the carving of the loin cloths and skirts, which arc usually the only clothing of the human figures.

During the century after Jaya varman's death, the capital was twice moved to other places, Roluos and Koh Ker on the lakes, for short periods before returning to Angkor with the accession of new kings. This century included the brief but extraordinary rule of one of the major kings of Angkor, Indravarman I. Much of what we know about him comes from the deciphered stone inscriptions on his temples. Between the year 877 and his death in 889, Indravarman organized the digging of a big artificial lake at Roluos to collect the flood water each year, then the spread of a network of irrigation channels to bring it to the rice fields of the town and a wide region around it. Many of the hydraulic techniques of the Khmer had been inherited from the old Funan kingdom. But the Funancse had been concerned to drain the soggy land of the lower Mekong delta, so the problems of irrigating the middle basin of the plain were new. Indravartnan's great religious monument, the famous Bakong temple-mountain, is at Roluos, several kilometres south-cast of Angkor, but his son Yasovarman brought the royal scat back to the city.

Yasovarman was very much his father's son. He brought the storage tanks of the Khmer irrigation system to the edges of Angkor itself and he built the first of the large temple-mountains in the city as his personal tribute to Shiva and monument to himself. This should not be seen as any sort of vanity, for just as the Inca kings each built a palace that in time became his mausoleum, each Khmer monarch-ruler of Kambuja, as Cambodia was called in the inscriptions from this period onwards-had a duty to build a new temple mountain for the god. On top of the step-pyramid representing the mountain, there was a shrine in which the god dwelt, though there might be a tower above this sanctuary in some architectural types. The god himself, usually Shiva, was represented by a statue or a linga in the shrine-this would ensure his actual presence for the benefit of king and people. Furthermore, since the king was a personification of the god, the temple-mountain was the monarch's own monument for ever. When the king died, his body would be cremated and the ashes buried in his temple. Oddly, it has not been established where the ashes were normally placed within the temple, but it seems that they may have been deposited beneath the god's statue.

An inscription on the stele (sculpted stone) known as the Sdok Kak Tmon tells us that Yasovarman founded a royal city `and led the god-king from Hariharalaya [his father's foundation] to this city. There His Majesty erected the central mountain, and the Lord of Shivasrama placed a holy linga in the centre.' That city of Yasovarman was the area between the much later monuments of Angkor Wat and Angkor Thorn. The temple-mountain at its centre was Phnom Bakheng, started in A.D. 893 and finished by the tune of Yasovarman's death in the year 900. The natural hill on which it was built was probably already a sacred mound of the Khmer and may well have determined the place where the king settled his capital. He carried out large public works around it before he started to build his temple.