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
Yoruba people ancient times, most of these cities were fortresses, with high walls and gates. Yoruba cities have always been among the most populous in Africa. Archaeological findings indicate that Òyó-Ilé or Katunga, capital of the Yoruba empire of Oyo (fl. between the 11th and 19th centuries CE), had a population of over 100,000 people (the largest single population of any African settlement at that time in history). For a long time also, Ibadan, one of the major Yoruba cities, was the largest city in the whole of Sub Saharan Africa. Today, Lagos (), another major Yoruba city, with a population of