Across Hausaland a network of walled city-states (the birni) such as Kano, Katsina and Zaria anchored the southern terminus of the trans-Saharan trade in leather, cloth, kola and salt. Each city was enclosed by great earthen ramparts pierced by gates; Kano's walls ran for kilometres around markets, dye-pits, the emir's palace and dense residential wards organised by craft and lineage. The cities were centres of Islamic learning, manufacture and long-distance commerce, governed by an emir and an elaborate court bureaucracy. Kano's famous indigo dye-pits supplied cloth across the region.
The Hausa birni is a durable indigenous model of the fortified West African commercial city, linking craft production, Islamic governance and Saharan trade in a planned walled form.