Krak des Chevaliers (also Crac des Chevaliers, "fortress of the knights" in a mixture of Arabic and French) was the headquarters of the Knights Hospitaller in Syria during the Crusades. The original fortress had been built in 1031 for the emir of Aleppo. It was captured by Raymond IV of Toulouse early in 1099, during the First Crusade, but was abandoned when the Crusaders continued their march to Jerusalem. It was reoccupied again by Tancred, Prince of Galilee in 1110. Raymond II, count of Tripoli, gave it to the Hospitallers in 1144.
The Hospitallers rebuilt it and expanded it into the largest Crusader fortress in the Holy Land, adding an outer wall 30 meters thick with seven guard towers 8-10 meters thick, to create a concentric castle.
The picture shows a part of this massive outer wall. Archers and smaller Trebuchets are trying to fight back the attacking Mameluks of Sultan Baibars.