The widespread construction of castles in Britain began as soon as Duke William of Normandy set foot on the shores of southern England in 1066. The castles that were constructed in the ensuing centuries, and whose ruins still scatter the British countryside today, provide us with an enduring record of the needs and ambitions of the times. But the essence of the medieval castle--a structure that is equal parts military, residential, and symbolic--reveals itself not only through the grandeur of such architectural masterpieces as the Tower of London, and the imposing nature of such royal...