Im surprised no one's said this yet but if you build up your army, declare war on someone, and YOU talk to them (not visa versa) you can sometimes offer a peace treaty for one of their citys.
eg.(xxx being human player, yyy being enemy city)
XXX wants: Peace treaty, YYY.
XXX offers: Peace treaty.
you can't trade for an enemy city any other way.
eg.(xxx being human player, yyy being enemy city)
XXX wants: Peace treaty, YYY.
XXX offers: Peace treaty.
you can't trade for an enemy city any other way.