It occupies the highest point of the Mahdia peninsula and was built at the end of the 16th century to host the Ottoman military garrison. This fort was probably built on the ruins of Fatimid ruler El-Mehdi's palace, of which some vestiges i.e. its base, were found below the north-west bastion. It is a large irregular rectangular building, flanked by three bastions at the northwest, southwest, and southeast corners. The two gates in the main façade of the fort, which are recent and date back to the colonial period, were created when the monument was transformed into a prison. Besides, the only 16th -century gate is located immediately to the left of the south-west bastion entrance. The jambs and the semi-circular arch of this entrance are decorated with diamond-shaped bosses. A inscription in Arabic lettering that mentions the date of construction of this fortress as 1595 can be seen above the lintel. This primitive door opens onto a vaulted vestibule whose walls comprise on both sides a recess, decorated from above with shell motifs. Inside, a small mosque occupies the south-east wing of the main courtyard of the 'Borj al Kabir'. A staircase leads to the terrace of the western wing of the fort, which is partly occupied by rooms that were probably built during the colonial period. The curtain walls of the northern and eastern wings of the fort are made up of fill between two masonry walls. To the south, the fort is defended only by a 2m wide wall, at the top of which there is a pedestrian walkway. From its upper part, this fortress offers a panoramic view overlooking the peninsula, embracing the whole city. The end of the façade of the south-west bastion is decorated with two bas-reliefs, depicting on the left a lion topped by a crossbow, and on the right only a crossbow. This is probably a reused Fatimid bas-relief since the lion represents the emblem of the city of al-Mehdi.