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 al-Salih into his ranks. By doing this, he hoped to secure the support of Yoruba Muslims and volunteers from the Hausa-Fulani north in keeping Ilorin independent. Torn by internal struggle, Oyo could not defend itself against the Fulani. Oyo-Ile was razed by the Fulani Empire in 1835 and the Oyo Empire collapsed in 1836. To this day, the Illorin traditional ruler is an emir, whereas in the rest of Yoruba towns the rulers are called "oba" or "baale" (Baale or Baba Onile meaning "father of the land" or "lord of the land"). After the destruction of Oyo-Ile, the capital was