The most famous medieval wars of European expansion, the Crusades, were originally military expeditions sponsored by the papacy for recovery of Christian sites in Palestine. The Crusades also provided land and opportunity for poor and restless knights. Castles were thus built by an alien aristocracy in a hostile environment to provide shelter and to maintain control over the surrounding countryside. After a sketch of the literature and of fortifications before the crusades, Kennedy (history, Univ. of St. Andrews, Scotland) explores the evolution of castle styles, siege techniques, and defensive technologies, relying on the evidence of both Western and Muslim chroniclers and of archaeology. Although this old-fashioned history is attractively written and extensively illustrated, greater attention to social and environmental conditions would have given a fuller picture. For research and general collections.