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 northern Jordan 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 Jordan and 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 (wikipedia)
In 1260 AD, the Mongols destroyed sections of the castle, including its battlements. Soon after the victory of the Mamluks over the Mongols at Ain Jalut, Sultan ad-Dhaher Baibars restored the castle and cleared the fosse. The castle was used as a storehouse for crops and provisions. When Izz ad-Din Aibak was appointed governor, he renovated the castle as indicated by an inscription found in the castle's south-western tower.