Kilwa Kisiwani is a small mangrove-fringed island off the southern Tanzanian coast, with the ruins of one of the great medieval cities of the Swahili world scattered across it. The Great Mosque, with its 16 domed bays still standing, was the largest mosque in sub-Saharan Africa when it was built around 1300. The Husuni Kubwa palace complex, a kilometre east, is a 14th-century Sultan's palace with an octagonal bathing pool and elaborate masonry that survives because it was so isolated.
The island is reached by small boat from the modern town of Kilwa Masoko, a journey of about 15 minutes. Permits are required (cheap; bought from the antiquities office in Kilwa Masoko); a local guide is essential — the ruins are extensive and unsigned. Allow a full day. Kilwa is a UNESCO World Heritage Site, but visitor numbers remain tiny.