As was pointed out in chapter 1, castles as they existed in the west, or even in Byzantium, were hardly known in Muslim Syria at the beginning of the twelfth century. Perhaps surprisingly, the arrival of the Aggressive Crusaders does not seem to have inspired defensive fortifications on the Muslim side, though the major earthquakes of 1157 and 1170 may have destroyed much of the evidence. Nur al-Din (1146–74) rebuilt the city walls of Damascus and Horns and the curtain wall of the citadel at Aleppo, and added a tower to the exterior of the cavea of the Roman theatre at Bostra. His work is very much in the twelfth-century style; the towers are small and comparatively far apart (though he did construct round towers on the city walls of Damascus and Horns), and the defences, both arrow slits and at the wall head, were comparatively simple.

Nur al-Din also rebuilt or repaired a number of fortresses along the marches between the Crusader Principality of Antioch and the territories of Aleppo. Among these were the fortifications on the tell at Harim, which must have resembled Aleppo in their time but which have now almost completely vanished, and the tell of Qal' at al-Mudiq. This was the citadel of the ancient city of Apamea which was taken by the Crusaders in 1106 and retaken by the Muslims in 1149. As at Harim, the curtain wall, embellished with rectangular towers, circles the top of the tell but we cannot really be sure how much of the building dates from the time of Nur al-Din and how much is earlier or later.