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 on 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.
After Usama's death, the castle was enlarged in AD 1214-1215 by Aibak ibn Abdullah, the Mamluk governor. He added a new tower in the southeast corner and built the gate.
The castle lost its military importance after the fall of Karak in AD 1187 to the Ayyubids. In the middle of the 13th century AD, the castle was conceded to Salah ad-Din Yousef ibn Ayoub, King of Aleppo and Damascus, who restored the northeastern tower and used the castle as an administrative center.
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.
During the Ottoman period, a contingent of fifty soldiers was set inside the castle. During the first quarter of the 17th century, Prince Fakhr ad-Din al-Ma'ni II used it during his fight against Ahmad ibn Tarbay. He supplied the castle with a contingent and provided provisions and ammunition. In 1812, the Swiss traveller Johann Ludwig Burckhardt found the castle inhabited by around forty people.
Two major destructive earthquakes struck the castle in 1837 and 1927. Recently, the Department of Antiquities of Jordan has sponsored a program of restoration and consolidation of the walls and has rebuilt the bridge over the fosse.