Roluos is a Cambodian modern small town and an archeological site about 13 km east of Siem Reap along NH6. Once it was the seat of Hariharalaya, first capital of Khmer Empire north of Tonlé Sap (as the first capital in the strict sense of the term could have been Indrapura, identifiable with Banteay Prey Nokor).
Among the "Rolous Group" of temples, there are some of the earliest permanent structures built by Khmer. They mark the beginning of classical period of Khmer civilization, dating from the late 9th century. Some were totally built with bricks, others partially with laterite or sandstone (the first large angkorian temple built with sandstone was possibly Ta Keo). At present it is composed by three major temples: Bakong, Lolei and Preah Ko, and tiny Prasat Prei Monti. At both Bakong and Lolei there are contemporary Theravada buddhist monasteries.
Information based on http://en.wikipedia.org/wiki/Roluos_%28temples%29