The temple complex of Angkor Wat in north-west Cambodia was built in the early 12th Century for King Suryavarman II.
Reconstructions suggest the one-time capital of the Khmer empire covered an area of 3,000 sq km (1,150 sq miles).
A new study suggests that the temple and surrounding settlements could have supported half a million people.
The entire area is criss-crossed by irrigation channels, now thought to have supported intensive rice agriculture.
The rapid growth of the city may have caused environmental problems that led to the collapse of the civilisation.
The ruins are now a World Heritage Site and are visited by thousands of tourists every year.