This is a temple called Phnom Bakheng. It is located on top of a hill and so, it does require, possibly, a 15-minute hike from the bottom.
Due to its altitude, a visitor has a good view of his surrounding. He can see Angkor Wat as well as an ancient reservoir.
As you can see, they are wooden structures raised to support some part of the temples. It is falling apart.
This particular temple was built in the 9th century when the Khmer empire moved it capital from Roulos, some 20 or 30 miles away from Siem Reap, to here.
And like most temples, Phnom Bakheng was built to replicate Mount Meru. The replication is less impressive than Angkor Wat, but Phnom Bakheng is still amazing. The view from the top is something to die for and Angkor Wat cannot match that.