A glimpse of the donjon.
From the Enc. Britt.: "The keep, or donjon, was the focal point of the castle, to which, in time of siege, the whole garrison retired when the outer works had fallen; it was therefore the strongest and most carefully fortified part of the defenses. It had a well; contained the private apartments, offices, and service rooms; and held all the appointments necessary to sustain a long siege. Often the keep stood in line with the outer line of defenses, so that while one side looked toward the bailey (or succession of baileys) commanding the operations of the defense there, the other side commanded the field and the approaches to the castle. The side of the keep exposed to the field also presented a line of escape."