The name 'Ajlun goes back to a Christian monk who lived on this mountain in the Byzantine Period. The castle stands on the ruins of a monastery, traces of which were discovered during archaeological excavations.
The castle has been the nucleus of a settlement which has grown to become the present town of Ajloun. The castle's developing faubourg led to its second name, Qal'at ar-Rabad, "the castle of the faubourg" or "the castle with the suburbs". This name still resonates in the surname of a large and reputable Christian family owning most of the agricultural lands in the direct vicinity of the castle until this day, the Al-Rabadis.
The fortress was built by Izz al-Din Usama, a commander and nephew of Salah ad-Din al-Ayyubi (Saladin), in AD 1184-1185. According to Saladin's historian Baha ad-Din ibn Shaddad, the fortress was primarily built in order to help the authorities in Damascus control the Bedouin tribes of the Jabal 'Auf. These enjoyed enough autonomy as to ally themselves to the Crusaders, and had at one point set up a 100-tent camp next to the Hospitaller castle of Belvoir on the opposite side of the Jordan Valley. As such, Ajlun Castle is one of the very few Muslim fortresses built by the Ayyubids to protect their realm against Crusader incursions, which could come from Beisan or Belvoir in the west and from Karak in the south.
From its location, 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-Yabis), and protected the communication routes between southern Jordan and Syria. It was built to contain the progress of the Latin Kingdom, which with the Lordship of Oultrejourdain had gained a foothold in Transjordan, and as a retort to the castle of Belvoir a few miles south of the Sea of Galilee. Another major objective of the fortress was to protect the development and control of the iron mines of Ajlun.
The original castle had four corner towers connected by curtain walls and a double gate. Arrow slits were incorporated in the thick walls and it was surrounded by a moat averaging 16 meters (about 52 feet) in width and 12–15 meters (about 40–50 feet) in depth.
After Usama's death, the castle was enlarged in AD 1214–15 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 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.
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