The Songhai Empire controlled the production and supply of Salt. This was a very important commodity which was known as white gold at the time.
At its peak the Songhai Empire was the largest and richest Empire in Africa and one of the most powerful in all of the Muslim world.
The cities of Timbuktu and Djenné became importing trading centers and remained one of the most important cities in Africa until the empire's demise.
Apart from salt, the empire grew rich due to its control over vast supplies of Gold, slaves, kola nuts, leather, and dates.
Eventually, the huge riches of the Empire attracted attention from other leaders of the region. In the last 20 years, the kings of Songhai were in constant feud with multinational armies which took a political and economic toll on the Empire.