Krak de Chevaliers () is a castle in Syria. The castle is on a hill and its position means that it can control the nearby valley and two Roman roads. The castle's builders were Crusaders. Between 1144 and 1271 the Knights Hospitaller used the castle. Because scholars have studied the castle and have restored the buildings, people say that Krak de Chevaliers is the best castle that the Crusaders built. The military design of the castle is complex and shows the last development of fortification by during existence of the Crusader states. It is one of the first "concentric castles" (castles with concentric layers of walls). Krak de Chevaliers was an important development in the design of castles, and many other castles' builders copied its design. No army in the Middle Ages ever took the castle by force. UNESCO has named the castle a World Heritage Site. 