How did the Kongo kingdom become rich?

By the early 15th century these communities had grown in wealth and size to form a loose federation centred on one kingdom, led by a king or Manikongo

Explanation:  Slavery and royal rivalries. In the following decades, the Kingdom of Kongo became a major source of slaves for Portuguese traders and other European powers. ... Despite its long establishment within his kingdom, Afonso believed that the slave trade should be subject to Kongo law.

Prospering on the regional trade of copper, ivory, and slaves along the Congo River, the kingdom's wealth was boosted by the arrival of Portuguese traders in the late 15th century CE who expanded even further the slave trade in the region.

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