The castle was far more than a walled and turreted fortress; it was an instrument of social control and the symbol of power, authority, and wealth. - Marilyn Stokstad

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.

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.

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