Angkor was the largest low-density urban complex of the pre-industrial world, a landscape of temples, embankments, canals and reservoirs (barays) supporting a dispersed population over hundreds of square kilometres. Tree-ring records from mainland Southeast Asia show decades-long megadroughts in the late fourteenth and early fifteenth centuries separated by unusually intense monsoons, and excavation by the Greater Angkor Project found the water network silted, its spillways ruptured and its channels patched rather than rebuilt. Political and religious change pulled elites toward maritime trade and Theravada Buddhism, and in 1431 an Ayutthayan army took the city; the court moved south toward the Phnom Penh region. Recent geoarchaeology indicates gradual decline over a century rather than a single catastrophic collapse, with the temples never fully abandoned.
Angkor is the classic case of an engineered hydraulic landscape whose scale and rigidity became its vulnerability once the climate moved outside the range it was built for, and it is now cited constantly in climate-adaptation and resilience planning. Its aerial and LiDAR mapping also forced a redefinition of what counts as a city, establishing low-density agrarian urbanism as a recognised settlement type.
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