I may be mistaken, but I think they also build the cities faster. I was playing an OCC and not only were they sending 5 axeman at a time into my cities ( I counted 9 barbarian units at once in my border), but they had knocked out the americans, and greeks. Usualy the AI moves right to my borders with their cities, but the AI was the first to get a city there. I was constantly hearing the names of barbarian cities being razed.