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 origin of human civilization, remains intact to this day. The urban phase of Ile-Ife before the rise of Oyo, c. 1100–1600, a significant peak of political centralization in the 12th century) is commonly described as a "golden age" of Ile-Ife. The oba or ruler of Ile-Ife is referred to as the Ooni of Ife. Ife continues to be seen as the "Spiritual Homeland" of the Yoruba. The city was surpassed by the Oyo Empire as the dominant Yoruba military and political power in the 17th century. The Oyo Empire under its oba, known as the Alaafin of Oyo, was active