Just as towers and crenellations indicated a building’s status, so the crenellated wall signified a castle in the visual arts and in that distinctive… - 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.

As castle design evolved, the great tower was eventually replaced by walled enclosures, which permitted more effective use of troops and better living conditions. The future of castle design lay with the curtain wall, that is, a wall “hung” like a curtain between towers, each of which functioned like a keep.

When a strong central authority protects borders and reduces internal crime, people have little need for fortified dwellings, although the rulers may build walls and towers to define legitimate residents and defend the country against external threats. When central authority breaks down, however, individuals are more likely to fortify their homes. The presence of castles in the landscape indicates a decline in stability and peace.