This huge fortress was built by Izz al-Din Usama, a commander and nephew of Salah ad-Din al-Ayyubi (Saladin), in AD 1184-1185. The fortress is considered one of the very few built to protect the country against Crusader attacks from Karak in the south and Bisan in the west. From its situation, the fortress dominated a wide stretch of the northernJordan Valley, controlled the three main passages that led to it (Wadi Kufranjah, Wadi Rajeb and Wadi al-Yabes), and protected the communication routes between south Jordanand Syria. It was built to contain the progress of the Latin Kingdom of Transjordan and as a retort to the castle of Belvoir a few miles south of the lake of Tiberias. Another major objective of the fortress was to protect the development and control of the iron mines of Ajlun.
The original castle core had four corner towers. Arrow slits were incorporated in the thick walls and it was surrounded by a fosse averaging 16 meters (about 52 feet) in width and 12–15 meters (about 39–49 feet) in depth.