biggest difference between king and the lower difficulties is that the AI is more aggressive and will create more militaryunits.
i can go an entire prince game been next to an AI and rarely have them declare war on me. on king its different, its always a constant war with my neighbours and there always throwing units at me. so yeah, king and up the AI and begins to make more units and is more aggressive. city-states build up a small army as well for defence.