The Great Mosque looks more like a fortress than a house of prayer, an impression which is heightened by the building's lack of a minaret. Built on the site of an old kasbah around 850, the building originally had a defensive role, protecting the port along with the nearby ribat. The courtyard dates back to the 9C, although the arcaded portico is more recent, dating from the 17C. Although the mosque is not open to the public, the prayer hall can be glimpsed through the open doors. Its 13 bays and vaults are supported by a forest of old columns and capitals and the central bay leading to the mihrab is crowned with two domes.