The remains of the temple atop Phnom Bakheng, as seen from the top of Angkor Wat.
Phnom Bakheng was one of the first temples built at the Angkor Wat complex, when King Yasovarman moved the Khmer capital from Roluos around the year 890. Unlike the later stone mountains at Angkor, Phnom Bakheng is a natural hill. It stood in the exact center of Yasovarman's new city, which was even larger than Angkor Thom, which was built much later.
The temple shows the first evolution of the design that would be perfected over later centuries at places like Angkor Wat. However, most people visit Phnom Bakheng for the view it affords of the other temples, especially Angkor Wat. In fact, the temple has become so popular for sunrise and sunset views that erosion has become a very serious problem. Don't be surprised if the temple is closed or allows only limited access when you get there.