This period the emergence of stone towns and increased coastal wealth and influence as trade expanded to the interior- as small towns grew to trade with and through the bigger ones, and also through the sea. There was more conversion to Islam and by the 11th century majority of coastal people were practicing Muslims. External chroniclers like al-Mas’udi described a place called ‘Qanbalu (Pemba, maybe?) with a Muslim ruling family, and in 1220 Yāqūt ibn ʻAbd Allāh al-Ḥamawī, (1179?-1229) described Mogadishu (remained so when Ibn Batutta visited in 1331) as a prominent urban center and also mentioned “Mkumbulu” in the Green island of Pemba. Zheng He’s circumnavigation of the Indian ocean between 1405 to 1433 included interactions with Swahili leaders in Malindi with whom he gave the finest imperial porcelain.
Building technology greatly improved in this period. The reef-coral buildings were replaced by coral rag structures. Coral was now used to build houses of certain Swahili elites and in some towns would have quite a number of such houses, and this usage of ‘stone’ was to later define Swahili townscapes
Coral rag consists of jagged blocks of coral. It is relatively soft and porous, but strengthens over time with exposure to the elements, making it well-suited for the coastal climate. Builders used coral rag for walls and foundations. Walls were constructed by bonding coral rag with lime mortar, made from burnt coral stone and shells, to create a strong and durable structure. Walls were finished with a lime plaster, often applied in thick layers (30-50cm). The porous nature of the lime plaster helps to keep walls dry in the humid coastal conditions.