From the 9th century onward, the Khmer Empire rising in what is now Cambodia built one of the largest and most complex urban settlements of the medieval world. The capital, Angkor, with its giant water reservoirs (called baray), canals and irrigation network, could by some estimates support between 750,000 and a million people — the largest urban area of the pre-industrial world.

The emblem of this civilisation is Angkor Wat, built by King Suryavarman II in the first half of the 12th century. With an area of about 200 hectares, it is the largest known religious structure. It was originally conceived as a temple-mountain dedicated to the Hindu god Vishnu; its rising central towers represent Mount Meru, the axis of the universe in Hindu cosmology. Over the centuries the structure became a Buddhist temple and has seen uninterrupted worship.

Kilometres of relief carvings on the walls depict scenes from Hindu epics, battles and daily life — among them the myth of the 'Churning of the Sea of Milk' is the most famous. As much as its scale, the structure's geometric precision and symmetry show the level of Khmer engineering.

Angkor was abandoned in the 15th century; the tropical forest swallowed the city, and for a long time only local people and monks remembered it. Its 'rediscovery' in the 19th century caused a great stir in the West. Today Angkor Wat is the national symbol of Cambodia and appears on its flag.