What is now known as the Roulos Group are three sites from a pre-Angkorian period that are preserved near the modern town of Roulos, some 13 km from Siem Reap. All three sites are temples dating from the late 9th century, and therefore preceed Angkor Wat by two and a half centuries.

Preah Ko was the first temple built on the site by Indravarman I when he established Hariharalaya as his capital. The temple has six main towers, three in front and three behind. The God-Kings of the Khmer after dying were thought to become forms of the Gods, and the temples at the front are dedicated to Kings in the form of Gods who were ancestors of Indravarman, and the three behind to their Queens.

Bakong was the State Temple of Indravarman, and is the first of the mountain-temples, which became very popular at Angkor. The foundation stele is still found and shows that the lingam was consecrated in 881 to Siva. The third temple (Lolei) is currently in very bad shape and is not shown in these photographs.