The Kingdom of Kongo organized Villages of small states along the Congo River, in which, the small states formed several larger principalities. Also, the kingdom maintained a centralized government with a royal currency system. The Kingdom of Ghana provided gold, ivory, and slaves for traders from north Africa and exchanged for horses, cloth, manufactured goods, and salt. Koumbi-Saleh, the capital city of Ghana, became a thriving commercial center. Ghana kings converted to Islam by the tenth century, but didn't force on others.