My biggest problem with the whole unit system, at the moment, is that you often build the unit then fortify them IN THE CITY!!!! Though I have no problem with having say 1 or 2 units fortified in a city, I feel that extra units should quickly diminish the peoples happiness (as it essentially reflects martial law!) Instead, using the existing 'rally point' system, I think that you should be able to set exactly where your units go after you build them (such as nearby airbases, naval bases and forts-or to a particularly strategic piece of turf!) Cities should become the 'home of last resort' for large forces and, in fact, most attackers should be encouraged NOT to attack cities immediately-and not with certain units-as the penalties for doing so should be quite high!
Hope that makes sense!
Yours,
Aussie_Lurker.