It did not assume its present form until an extensive renovation and expansion by the Ayyubids in the 13th century, using locally quarried basalt which makes the castle darker than most other buildings in the area.

The main entrance is composed of a single massive hinged slab of granite, which leads to a vestibule where one can see carved into the pavement the remains of a Roman board game.

The strategic significance of the castle is that it lies in the middle of the Azraq oasis, the only permanent source of fresh water in approximately 12,000 square kilometres (4,600 sq mi) of desert.

Qasr al-Azraq underwent its final major stage of building in 1237 CE, when 'Izz ad-Din Aybak, an emir of the Ayyubids, redesigned and fortified it.

So I established myself in its southern gate-tower, and set my six Haurani boys...to cover with brushwood, palm-branches, and clay the ancient split stone rafters, which stood open to the sky."

The postern gate was shut each night, "The door was a poised lab of dressed basalt, a foot thick, turning on pivots of itself, socketed into threshold and lintel.

Lawrence was told, "...the dogs of the Beni Hillal, the mythical builders of the fort, quested the six towers each night for their dead masters...their ghost-watch kept our ward more closely than arms could have done.