The circular stone wall of Krol Romeas
Generally, stone was only used in the construction of temples and other religious structures at Angkor. Other buildings, including palaces, were made of wood. This makes Krol Romeas unique among the ruins of the ancient city. It's clearly not a temple. A few people argue that the stone ellipse was a baray, or pond, but that seems very unlikely. The walls are too thick for such a small pond, and there appears to have been an inner wall which wouldn't make any sense for a water pool.
The suggestion that makes the most sense, given the layout of the place and the details you discover, is that Krol Romeas was an elephant corral, where elephants may have been bathed and prepared for riding. Slots in the inner face of the thick outer wall could have held posts, where elephants would be tied up to be boarded by the court nobles and generals. The location just outside the north gate of Angkor Thom, reinforces this hypothesis.
Find Krol Romeas is a bit of a trick. Leaving Angkor Thom via the north gate, the road curves around to the right. Keep a sharp eye out for the tiny sign on the left side of the road, opposite a small path which leads off the right side. The ruins of Krol Romeas is about 50 meters down the path.