Koh Ker is one of the ancient Khmer cities built by King Jayavarman IV in AD 921, when the centre of power was moved from Angkor. Within the new city, the King Jayavarman IV built the biggest Pyramid of his reign. In the city centre, he also had the largest reservoir known as Rahal constructed. Water management at Koh Ker was different from that designed at Angkor. At Angkor, water sources flowed naturally from the mountains of Phnom Kulen and Kbal Spean into the city. Koh Ker, on the other hand, is far from natural sources of drainage (rivers and streams) and water supply relied almost entirely on rainfall. This has led to debates among scholars about the type of urban planning and water management was designed and utilised at Koh Ker. This presentation provides details on new research at Koh Ker applying on-site survey and LiDAR, and how these techniques have aided in interpreting the design of Koh Ker, and the complex water management systems that were constructed to collect and supply the city of Koh Ker. It focuses on three lines of evidence: 1. the organisation of Jayavarman IV’s city in the 10th century and the plan of the royal temple Prasat Thom alignment with Prasat Kraham and the five Linga temples; 2. Prasat Chen, along with Prasat Banteay Pir Chean and Prasat Thom form the Trinity of temples at Koh Ker; and 3. and the concept, organisation of water management at sacred places associated with water at Trapeang An Khnar and the many different carved Hindu gods that surround Beong Khnar which allowed rainfall to travel into Rahal and then be shared to other reservoirs in the city.