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.

Ch. 2 : The Castle as Fortress : The Castle and Siege Warfare - Medieval castles (2005)