Ninth-century castles were relatively small and simple affairs designed to safeguard a relatively small number of people and intended as a refuge dur… - 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.

Castles continued to be the focus of economic activity as the center of an agricultural domain. Wealth continued to be measured in land and its produce. The only access the lord had to his wealth was to move from one estate to another consuming products from the harvests. Housing and feeding a household including retainers and servants required vasts amount of food and space for food preparation.

As they captured each territory, William and his men secured their camps with simple earth and timber defenses characteristic of northern Europe (...). These wooden structures have disintegrated, but sometimes their earthen mounds survive as rolling hills or picturesque elements in the landscape. Timber castles were especially useful to a warrior king like William the Conqueror who moved rapidly to bring new territories under control. The earth and timber castles could be built quickly, since the newly moved earth did not have to support great weight. Such structures were also relatively cheap because they required no specialized masons and expensive stone. Since the timber building tradition was widespread both in the British Isles and on the continent, the carpenters knew the building techniques. These castles were essentially towers and stockades; they provided garrison headquarters as well as residences.