This borj, also called Kasbah, is a fortress built at the end of the XVIth century on the site of the former Fatimid palace and is considered one of the finest specimens of Ottoman military architecture.

Built on a quadrangular plan and later endowed with corner bastions, the building is surrounded by strong walls originally with one entrance only (after it was turned into a prison, another access was added in the XIXth century). This door leads through a vaulted and elbowed passage, into a courtyard onto which various vaulted rooms open. In the south-eastern corner of the courtyard, an earlier oratory was conserved and integrated into the building.

From the wall-walk, converted to a terrace, there is a lovely view over the tip of the Cape Mahdia promontory and, closer to the monument, over the ancient harbour basin that certain specialists date to the Punic period.