Oyo Empire culture in Oyo where victory was obligatory and defeat carried the duty of committing suicide. This do-or-die policy no doubt contributed to the military aggressiveness of Oyo's generals. The Oyo Empire was the only Yoruba state to adopt cavalry; it did so because most of its territory was in the northern savannah. The origin of the cavalry is disputed; however, the Nupe, Borgu and Hausa in neighboring territories also used cavalry and may have had the same historical source. Oyo was able to purchase horses from the north and maintain them in metropolitan Oyo because of partial freedom from 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