The keep was usually a tower or tower-like structure used in time of invasion by the lord and his family. The people in the keep would wait for help or they would negotiate with the intruders. Since the castle was made for defensive means, most of the architectural elements within it are for defensive purposes. T he most important aspect of the castle were the walls surrounding the bailey. These altered throughout the Middle Ages to include improvements in defense.
Battlements go hand and hand with medieval walls.
Battlements refer to the alternating openings and upright slabs of stones used for defensive means. This made it easy for protectors of the castle to drop things on attackers while being protected. Also along these lines were arrow slits in the gates or towers along the walls. These were designed for archers to be able to shoot out, while invaders would have to have a near-perfect shot to hit the slit. T he tower-walls changed with time as well.