When a strong central authority protects borders and reduces internal crime, people have little need for fortified dwellings, although the rulers may… - Marilyn Stokstad

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.

When an attacking force laid siege to a castle, they used techniques and weapons not unlike those developed by the ancient Romans. First they surrounded the castle in order to cut off all avenues of escape and resupply. They also built a camp ringed by ditches and palisades to secure their own position. Then they built siege engines—great stonethrowing devices—which they hoped would break down the castle walls. Although the knights’ chivalric code gave pride of place in warfare to a charge on horseback with lance or to hand-to-hand combat with swords, military engineers skilled in the mechanics of offensive engines had to first break through the walls. To breach the walls the army used battering rams, various kinds of projectiles, and mines. In other words they tried to go through, over, or under the walls.

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.