Baphuon Temple, Angkor Thom
The Baphuon temple was built around 1060, before the royal city of Angkor Thom was built around it, and only recently has restoration work resumed. The Baphuon was a man-made mountain or pyramid made of stone. Originally the temple was topped by a bronze shrine according to Zhou Daguan.
The restoration of the Baphuon is something of a saga in itself. The French began a massive reconstruction effort around 40 years ago. They carefully disassembled the temple, carefully numbering all the stones and documenting everything. But in 1972 the were forced to leave due to the civil war. On their return, they found that all of their careful documentation had been destroyed, leaving them with a giant jigsaw puzzle, but without the picture on the box that tells you how to put it together. They eventually figured it out and started to put the temple back together. Work is only about half done, which means that the temple is largely off-limits to visitors.
Most visitors enter and leave via the east gate, next to the Terrace of the Elephants. However, you can easily walk here from the Bayon, and there's a nice path from the road up past the ponds in front of the Baphuon. In addition, you can leave through a gate in the wall on the north side of the temple, which gives you access to the royal enclosure with the Phimeanakas at its center.