From the outside, the Great Mosque of Kairouan is a fortress-like building with its 1.90 metres thick massive ocher walls, a composite of well-worked stones with intervening courses of rubble stone and baked bricks. The corner towers measuring 4.25 metres on each side are buttressed with solid projecting supports. Structurally given the soft grounds subject to compaction, the buttressed towers added stability to the entire mosque. Despite the austere façades, the rhythmic patterns of buttresses and towering porches, some surmounted by cupolas, give the sanctuary a sense of striking sober grandeur.