Wherever they went, William and his Norman lieutenants built castles. About 170 great vassals came to England with William. When the king rewarded his followers with grants of land, they also assumed the responsibility for its defense, so each built one or more castles. William and his men had several reasons for building castles. As hostile invaders they had to fortify their dwellings and camps in order to hold the territory and provide security for themselves. Their castles also secured borders and coastlines against other invaders and controlled the movement of people and goods at key transportation centers such as fords, bridges, and passes and along major roads.

Ch. 1 : The Great Tower : Norman and Early Plantagenet Castles - Medieval castles (2005)