Where are the Dowager's prisoners held in the novel, Wildwood (Wildwood Chronicles)?

Within the warren there is a huge cavern that serves as the prison. A large tree and its root system form the ceiling of the prison. From these roots, long ropes support wooden birdcages, large enough to hold a person. Each cage serves as a cell for prisoners. The cages hang about sixty feet above the floor which is covered with jagged rocks and the bones of former prisoners who apparently jumped to their deaths. The cages are accessed by way of a long ladder which is kept by the exit door. A single warden keeps watch over the prisoners.