The Sahelian kingdoms of Africa

The Sahelian kingdoms once stretched across grasslands (called the sahel) which bordered the interior side of the Sahara desert, gaining wealth from routes traveled by traders on camels and horseback. While bounded by the inhospitable desert to the north, Ashanti and Yoruba warriors well-adapted to life in dense forests were simultaneously effective obstacles to any expansion of these kingdoms southward. This produced a "stripe" of advanced city-states cutting across the heart of Africa as illustrated above. Wikipedia speaks in more detail on the Sahelian kingdoms.