The key to the economic might of the Khmer Empire from the 800s to the 1300s was?

Mekong–Tonlé Sap rice–fish symbiOSis

Annual flood-pulses from the Mekong deposited silt and fish into the Angkor plain, renewing fertile soil and supplying natural protein without fallow cycles. Artificial reservoirs (baray) and canal grids stored peak-season water for the dry months, enabling multiple rice harvests and high-density fish catches that fed the capital and generated large surpluses.