Krak des Chevaliers in Syria. It is an 11th century castle and was used in the Crusades. It was one of the first castles to use concentric fortification, ie: concentric rings of defence that could all operate at the same time.
The Siege of Acre took place in 1291 and resulted in the loss of the Crusader-controlled city of Acre to the Muslims. It is considered one of the most important battles of the time period. Although the crusading movement continued for several more centuries, the capture of the city marked the end of further crusades to the Levant. When Acre fell, the Crusaders lost their last major stronghold of the Crusader Kingdom of Jerusalem.