Excerpts from Books and Wikipedia
"The Crusaders began to build castles as soon as they arrived in the east. Krak des Chavaliers in modern Syria was built in 1131 by the Crusading Order of the Knights of St. John (Hospitallers). Built to withstand sieges, it had two concentric towered walls that were separated by a wide moat. It stood on a natural rocky outcrop, which was inaccessible and gave commanding views of the surrounding country side. It could garrison 2,000 men. The castle was held by the Knights of St. John until 1271, when it was captured by the Mamluk sultan, Baybars I." [DK Timelines, p. 186]