Originally posted by Trev
I play at Emperor level, if the AI suspects you are weak, they will attack to take your iron resource, it is necessary to adequately to defend all your cities and resources in the early game at that level, at lower levels it may be possible to get away with a weaker army.
Also the units are usually needed to maintain order in the city at that level, occasionally it may be possible to get 3 luxuries early and roaded to all cities so units in cities are not necessary, but this is rare