From The Grey House:
Vincent had established the oubliette when he bought the place. It was an old well. He had enlarged the reservoir and smoothed out the walls. There was one way in, a twenty-foot drop from the dungeon. Though old vampires at full blood could survive the fall and jump out, Vincent never threw anyone in the pit who was at full blood. Keith had not been fed in long enough that the young vampire would not be able to escape by jumping out. Also, the walls were too smooth to climb. In the reservoir was his other Judas: the child he had not told Keith about. It took care of the remains and made sure no one was able to leave.