The great basalt fort of Deir Al-Kahf is located on the north side of the highway that stretches from Mafraq to the Iraqi border and about 45 km from Qasr Azraq. Built in the 4th century and historically situated on the Strata Diocletiana between Azraq and Bostra, it is another purpose-built Roman fort, a site of considerable importance that primarily served as a sentry post. Like its famous neighbour Umm al-Jimal, Deir Al-Kahf is also constructed of black basalt, though the ruins here are not nearly as extensive.
An entrance has now been made in the west wall, but the original entrance, where a huge basalt gate is embedded in the ground, was in the east. The fort is about 60 m square with corner towers and two interval towers in the north and west walls. The walls are 1.5 m. (5 ft) thick and still rise in places to some 7.5 m. (25 ft). The curved line of the apse of a church added at a later stage can just be seen in the courtyard near the plastered cistern.
Around the courtyard were two storeys of some 100 rooms on two floors (three in the towers) which would have been adequate accommodation for a 500-strong cohort at least. In the south wall a small postern, now blocked, led to a walled and covered cistern. A Greek inscription found over the lintel here referred to the prefect Agrippa. There are other cisterns and pools found in the neighbourhood which provided an additional supply of water. The fort appears to have been abandoned in the late 5 century for unknown reasons.
It is certainly off the tourist routes but well worth a visit and If you decide to do so today, a local guide can be hired for a small amount (1 or 2 JOD) to show you around and tell more about the history of this fort.