The ribat at Sousse is both the oldest and most typical surviving example of the ribat typology as it existed in medieval North Africa. A series of small fortifications known as "ribats" were constructed along the North African coastline during the ninth and tenth centuries to accommodate both military and religious functions. Small garrisons of devout Muslim soldiers lived within the ribats and provided protection for their cities from maritime attacks. During times of peace, these volunteer warriors devoted themselves to their faith and served as religious teachers to the community. Ribats were typically simple in design and mostly unadorned, due to their principal function as a military fortification. The design of ribats such as the one at Sousse strongly influenced later madrasa design in the region, prefiguring the arrangement of multiple levels of small cells surrounding a central courtyard.

The height of ribat construction was limited to the first centuries of Muslim rule in the region. Though initially established by an Abbasid governor in 796 CE, the ribat at Sousse was demolished and fully reconstructed by Aghlabid caliph Ziyadat Allah I in 821 CE/AH 206. It is this ninth century structure that survives today, restored but largely unaltered.

The ribat at Sousse is located just inside of the northern perimeter of the Sousse médina, approximately 500 meters inland from the coast. The ribat is located fifty-five meters northwest of the Great Mosque (851 CE / AH 236), accessible by foot across the Place des Martyrs. Its northern edge faces the Boulevard Yahia Ibn Omar, although pedestrian access is limited by the ramparts encircling the old médina, which pass between the ribat and the road.

The ribat is square in plan, composed of four thickened walls enclosing a central courtyard. These walls measure thirty-eight meters along their exterior faces and fourteen meters high. The north-south axis of the structure aligns almost exactly with the north-south meridian. At the center of each exterior wall, a semi-circular tower projects two meters from the face of the wall and sixteen meters high. Circular towers measuring four meters in diameter and sixteen meters high are located at the northeast, northwest, and southwest corners. The southeast corner is fortified by a square bastion, which serves as the base for a watchtower with a circular shaft, surveillance balcony, and cupola. The watchtower was inspired by similar Abbasid minarets that were constructed during the late eighth century throughout Ifriqiya, the coastal region comprising modern day Tunisia, western Libya, and eastern Algeria. The tower rises to a height of approximately thirty-five meters, with the balcony located thirty-one meters above the ground.

The sole entrance to the ribat is located in the center of the south elevation, announced by a shallow porch. The six-meter-tall and two-meter-wide entrance portal is flanked by antique marble and granite columns and capitals from the Byzantine settlement which predated Aghlabid Sousse. The entrance porch encloses a small square hallway roofed by a groin vault, also likely a vestige of the city’s Byzantine and Roman past. In addition to its decorative features, the entry porch incorporates machicolations, slits in the floors of the upper level of the fort that allowed soldiers to drop rocks onto enemies attempting to gain access. The porch also featured a type of heavy gate known as a portcullis, which could be dropped to bar entry through the portal. This portcullis, the machicolations, the exterior arrow slits, and the narrowness of the single entry were all defensive strategies employed to create a formidable barrier against invading forces.

The covered hallway opens onto the ribat’s central courtyard, measuring approximately twenty meters wide east to west and sixteen meters deep north to south. The rectangular shape of the courtyard is due to the thickened depth of the south gallery relative to the enclosures that line the north, west, and east walls; the south gallery is thirteen meters deep while the other spaces are nine meters deep. On the ground level, the galleries are partitioned into thirty-three small cells, which housed the residents of the ribat. Due to its compact size, the ribat was originally designed to support a garrison of fifty or fewer members. As there were no communal spaces apart from the prayer hall and central courtyard, residents of the ribat prepared personal meals and studied in their small cells. Such rooms line the interior of the east, north, and west walls, with a single bay of arcades facing the courtyard in front of the enclosed spaces on each side. Instead of private cells, two bays of arcades border the south entry wall.

Along the south side of the courtyard are two staircases that allow access to the upper level of the ribat. These unroofed stone staircases both begin at the center of the south elevation, just beyond of the entrance hallway, and rise parallel to the arcades towards the southeast and southwest corners of the ribat. The staircases open onto an uncovered ring of walkways located atop the arcades of the east, north, and west elevations. A second level of tiny rooms surround the central walkway along those three elevations, while an enclosed prayer hall is located atop the arcades of the south elevation. The prayer hall is two bays deep and eleven bays wide, facing the qibla niche located in the south wall. Arched entrances are located within each of the eleven bays, allowing praying soldiers easy mobility to other parts of the ribat if needed in cases of attack.

The entire structure of the ribat was constructed in stone, primarily in a coursed ashlar pattern. The ceilings of the small cells on the ground level are composed of stone rubble. The exterior walls are topped by one-meter-high defensive stone merlons with central arrow slits and semi-circular profiles. The exteriors of the southern wall and adjacent towers feature a continuous cornice of scalloped corbelling just below the merlons, as well as arrow slits accessible from the first level prayer hall.

One notable structural feature is the small freestone dome located above the entrance porch. The circular dome rises from an octagonal base supported by squinches. This dome is the oldest of its kind in existence, and a precursor to the dome of the nearby Great Mosque of Kairouan (817-838, 856-863).

Though the ribat features extremely limited ornamentation, there is a commemorative inscription located above the door that provides access to the watchtower. This carved inscription is the oldest in Tunisia, listing the date of reconstruction of the ribat as 821 CE / AH 206 and its patron as Ziyadet Allah I.

During the centuries after its construction, the ribat was used variously as a military base or as a caravanserai for religious pilgrims, depending on the political climate of the era. The ribat at Sousse was damaged during the 1943 shelling of the city during the North African campaign of the Second World War. It was subsequently restored between 1951 and 1953. Though no longer in use as a military center, the ribat is frequented today by pilgrims, architectural historians, and tourists.

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