What "superior skill" did warrior from Ghana have

An ancient African kingdom we call Ghana existed in West Africa between the Niger and the Gambia Rivers from about 300 CE to about 1100. The rivers were important to Ghana because its economy was based on trade. Before the modern age, rivers were the fastest way to carry goods. Ghana became wealthy by collecting taxes from traders who passed through the kingdom. The people called their nation Wagadu; we know it as Ghana because that was the name of their war chief.