Rolous Group was built by King Yasovarman I 889 to 910 AD. It was a group of temples dedicated to Jayavarman II. It has 3 groups of temples Lolei Preah Ko
and the largest one, Bakong
(you can visit all of them in a tuk tuk for less than US$10.
Entrance: US$ 40 (can be bought from Angkor Ticket Booth north of Siem Reap)
You can reach there by tuk tuk (I paid US$10/half day including the 3 temples in Rolous Group). The Temple was located south of Siem Reap