I love the idea of choke points outside town.  
I never thought about the inability to shoot down from a hill top.
Perhaps the 37s on the hills tops and maybe AT near the bridges into town?  
Thing is this...
V Bases have autos and manned guns.  Even when you drop hangers, it is a challenge to knock down all of the manned guns,  Same as port.  So where ever there is a map room, should have some manned gun system IMHO.  Then you have to account for not only the 8 AAAs but also up to 7 manned guns.  
Currently, a single set of lancs can WF a town in a single pass if you know what you are doing.  That is not an exaggeration.  Then you have a single fighter make 2 passes and you are deacked.  
Adding manned guns in town adds to the difficulty of taking an airfield.  Plus even when the manned guns are down for a period of time, the manned guns will pop after a bit.  And it would stop someone going in and deacking a town, getting it white flagged, then bailing and coming back with troops unopposed.