Oyo Empire metropolitan Oyo. This may have been fine in the 15th century when Oyo controlled only its heartland. But to make and maintain farther conquest, the structure underwent several changes. Oyo maintained a semi-standing army of specialist cavalry soldiers called the "Eso", the "Esho" or, formally, the Eso of Ikoyi. These were 70 junior war chiefs who were nominated by the Oyo Mesi and confirmed by the Alaafin of Oyo. The Eso were appointed for their military skill without regard to heritage and were led by the Are-Ona-Kakanfo. After Oyo's return from exile, the post of Are-Ona-Kakanfo was established as the
Oyo Empire in the present Oyo died in 1905. See List of rulers of the Yoruba state of Oyo. During the colonial period, the Yorubas were one of the most urbanized (living in city-like areas) groups in Africa. About 22% of the population lived in large areas with population exceeding 100,000 and over 50% lived in cities made up of 25,000 or more people. The index of urbanization in 1950 was close to that of the United States, excluding Ilorin. The Yoruba continue to be the most urbanized African ethnic group today. Old Oyo linked cities such as Ibadan, Osogbo, and Ogbomoso,