Crac des Chevaliers (Castle of the Knights), possibly the greatest surviving example of medieval military architecture, dominates the area of Qal’at al-Hisn, Syria, near the Lebanese border. It was built by the Knights of the Hospital of St. John—the Hospitallers—a military order dedicated to providing medical assistance to pilgrims to the Holy Land. Constructed high atop a hill on the site of a former Kurdish stronghold, it remained in Crusader hands from 1142 until it fell to the forces of the Mamluk Sultan Baybars in 1271 during the Eighth Crusade. The castle, with its two concentric fortifications embellished with arrow-slits for archers, machicolations (balconies and chutes for dropping rocks or for pouring liquids on assailants) and a moat to the south, could accommodate a garrison of 2,000 men. In the accompanying article, author Adrian Boas surveys the history of the Crusades and describes the formidable castles the Crusaders erected in the 12th and 13th centuries.