The earliest temple monuments to have been built in  the Angkor region, the Roluos Group borrows its name from the small town of Roluos, 8 miles (13 km) east of Siem Reap. The temples mark the site of Hariharalaya, the first Khmer capital established by Indravarman I (r.877–89). Three main complexes can be found here. To the north of Highway 6, en route to Phnom Penh from Siem Reap, is Lolei. Founded by Yasovarman I (r.889–910), this temple stands on an artificial mound in the middle of a small reservoir, and is based on a double platform surrounded by a laterite wall. The four central brick towers have surprisingly well-preserved false doors and inscriptions.

To the south of Lolei stands Preah Ko, meaning the Sacred Bull. Built by Indravarman I, to honor his parents as well as Jayavarman II, the founder of the Khmer Empire, this temple was dedicated to the worship of Shiva. The main sanctuary consists of six brick towers resting on a raised laterite platform. Close by are three statues of the sacred bull Nandi, for whom the temple was named, which are in a remarkably good condition. The motifs on the lintels, false doors, and columns are also well preserved. They include kala, mythical creatures with grinning mouths and large bulging eyes; makara, sea creatures with trunk-like snouts; and Garuda. The temple sits resplendent in its serene rural setting. Beyond Preah Ko, the huge mass of Bakong, by far the largest of the Roluos Group, is well worth a visit. Originally dedicated to Shiva in AD 881, the temple has since become a place of worship for Buddhists. More than a 1,000 years ago, it was the central feature of Hariharalaya, as a temple- mountain representing Mount Meru, the mythical abode of the gods. Approached by a pathway that is protected by a seven-headed naga and flanked by guesthouses for pilgrims, the mount rises in four stages, the first three of which are flanked by stone elephants at the corners. At the summit rests the square central sanctuary, with four levels and a lotus-shaped tower rising from the middle. The mount is surrounded by eight massive brick towers that feature finely carved sandstone decorations.