Islamic Empire had diverse people. Thier prophet Muhammed said the Arab isn't different from the none Arab the black isn't different from the white we are all the same in God's eyes. Medieval European cities had King/Queens. Muslims had caliphates. Men who were chosen by the people to be successors of Muhammed. The fourth Caliphate Omar said "Oh people if I don't do what the prophet and God has asks me to do then do not follow my lead".