At the beginning of the eleventh century the motte was the most widespread form of castle in northern Europe (except the British Isles and Scandinavia).
The castle was primarily the residence of the lord, but it eventually functioned as a treasury (place where valuable items are kept), armory (storage facility for weapons), and center of local government. Lords first built castles in order to defend their fiefs. The original castles were little more than hills surrounded by ditches and topped with wooden forts. These forts were known as motte and bailey castles. “Motte” was the original term for a “moat,” a deep ditch that surrounded a castle to provide protection from invaders. The bailey was a hill constructed with the dirt that had been removed in digging the motte. Usually located at the fringes of a territory, a castle could be built quickly and was cheap enough to abandon in a hurry.
Motte and bailey castles were only built as temporary measures. As quickly as possible, the Normans began constructing permanent fortresses in stone. In these, the keep, an enormous tower incorporating living accommodation, a chapel, and storerooms, was the most important feature. The White Tower in the Tower of London is a perfect example. A bailey, consisting of a large, well-defended gatehouse and a series of square towers joined by a high wall named the curtain, was next to be built. The whole was then surrounded by a dry ditch or wet moat, crossed by a drawbridge. Experience revealed that the angles of the square towers were vulnerable to siege catapults, so round towers that provided a deflective surface replaced them.