Spean Thma (“Stone Bridge”), was built in the 16th century, where the road from the Royal Palace to Ta Keo crosses the river and had 18 pillars.
The building material was sandstone blocks that were taken from Angkor Thom, some of them are with reliefs.
The ruin of the bridge shows us how the bed of the river was deepened as the water ran faster and washed away the foundation of the pillars. The River moved to the east. Now it runs several meters deeper.
King Jayavarman VII had the area of rice cultivation and artificial irrigation extended to the north, off the plain and into the slopes of the Phnom Kulen and Kbal Spean where the water runs faster.
The outcome was an increase of erosion which gradually destroyed the system of irrigation.
There was also a draught in the 14th century.
Jayavarman’s building program put too much strain on the capital. Inner tensions cropped up as the Hindus refused to accept the supremacy of Buddhism.
The royal court moved to the south of Cambodia. By and by, people abandoned Angkor.
Angkor declined by itself and hardly by forces from outside.
The Siamese (Thais) are said to have conquered Cambodia in 1431. This is not proved. It’s mere fiction. History knows next to nothing about the 14th and 15th centuries. (“Siem Reap” means Defeat of the Siamese).
Theravada Buddhism spread under Siamese influence.
In the 16th century, Theravada Buddhist monks destroyed the five big towers of the Shiva temple Phnom Bakheng, the unique pyramid of the Shiva temple Baphuon, and die central sanctuary in the Vishnu temple, Angkor Wat.