Constant warfare, especially against the Muslims, gave rise to a new type of military man—one who combined the character and role of both monk and warrior. These knights, organized into military orders, served officially under the Pope but were essentially independent. Their grand master was both an abbot and a general. They lived under a modified Cistercian rule, and they took monastic vows of obedience, poverty, and chastity. As monks, in theory they owned nothing; for example, their horses and armor were loaned to them by the order. In practice they became a wealthy and often arrogant standing army. Having studied Byzantine and Muslim castles and warfare, they built huge castles that changed castle design in Europe.

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