Boris Gudenuf
Deity
A ground AA Addition, representing the combination of heavy and light gun batteries (and later SAMs), RADAR, Acoustic and human air spotters, command and control nodes, et al should be available as an Upgrade at pretty much the same Tech as the (Heavy) Bombers. But it should be an expensive Upgrade: in WWII Germany is estimated to have had over 1,000,000 men and 1/3 of their artillery production dedicated to ground air defense by 1944, in addition to a huge share of their 'artillery' ammunition production.If there is going to be an AA unit, I would put it on the tech tree as an automatic upgrade to walled districts. This would also lessen the imbalance in games where the player gains an air advantage, where in 6 the steamrolling began. Interestingly the AI in 6 had the capability (buried somewhere) to conquer cities using bombers and land units, getting into a rhythm of capturing a city every turn or two, but I only saw it deployed in an all-AI game with various tweaks to their decision weightings. If not actively disabled in the coding of 7, I suspect that if the AI builds the units, they will use them.
Representing the different nature of ground versus air defenses, though, I would make the Upgrade available to any Urban District. It would provide a significant defense against any kind of air attack, and a small addition to any defense against ground attack (batteries, battalions and regiments of 20 - 37mm automatic cannon and 75 - 128mm high-velocity cannon are not insignificant defensive measures!).
Assuming, of course, all of that is warranted by gamer interest . . .