Wikipedia
Bakong is the first temple mountain of sandstone constructed by rulers of the Khmer empire at Angkor near modern Siem Reap in Cambodia. In the final decades of the 9th century AD, it served as the official state temple of King Indravarman I in the ancient city of Hariharalaya, located in an area that today is called Roluos.
The structure of Bakong took shape of stepped pyramid, popularly identified as temple mountain of early Khmer temple architecture. The striking similarity of the Bakong and Borobudur temple in Java, going into architectural details such as the gateways and stairs to the upper terraces, suggests strongly that Borobudur was served as the prototype of Bakong. There must have been exchanges of travelers, if not mission, between Khmer kingdom and the Sailendras in Java. Transmitting to Cambodia not only ideas, but also technical and architectural details of Borobudur, including arched gateways in corbelling method.