The building is divided into three stories of which the lowest is a very tall basement level with several utilitarian windows and a doorway giving onto the exterior court. Internally, the basement is robustly vaulted to support the chapel. Above is the main space, the Sistine Chapel, the vaulted ceiling rising to . The building had six tall arched windows down each side and two at either end, several of which have been blocked. Above the vault is a third story with wardrooms for guards. At this level, an open projecting was constructed, which encircled the building supported on an arcade springing from the walls. The gangway has been roofed as it was a continual source of water leaking in to the vault of the chapel.