Stone towers appeared early in the Loire River valley. The massive ruin at Langeais, recently dated 992, was once a broad tower with four corner turr… - Marilyn Stokstad

Just as towers and crenellations indicated a building’s status, so the crenellated wall signified a castle in the visual arts and in that distinctive medieval sign language known as heraldry. The heraldic symbol of the kingdom of Castile, for example, consisted of a wall and three crenellated towers. This simple composition was easily recognized and reproduced.

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.

Warfare had become endemic in eleventh- and twelfth-century Europe. Castle building used up the resources of the land as every landholder from the king and great nobles to the small landholders fortified their dwellings. Constant skirmishing, brigandage, and open warfare at home and abroad meant that people poured vast resources into training and equipping warriors and building castles and siege machines. The motte and bailey castle with its great tower, as the keep or donjon is called in medieval documents, was admirably suited as a defense against local skirmishes.