I believe its been mentioned many times, but perhaps forts should get their own line of promotions...
It could be handled in different ways, but just one that pops to mind is making a separate (additional) Defense Slider that works much the same way as espionage. You build points via buildings and specialists the same way as espionage, and perhaps by spending cash into your slider (but doesnt necessarily have to even be a slider)
Castles +2Defense, Wall+1, MilitaryAdvisor+3, GGeneral spent+1000, GreatWall+8, etc......... Any manner of points for balance.
These point could be spent to upgrade promotions in forts and city defenses.. Something like Defense 1 through 5, ZOC 1 through 5, Airbase 1&2 (number of planes 4&8), Dispensary 1&2 (healing), Bunker 1&2 (defense from bombs&Artillary), AA 1&2 (chance for intercept)...
The list and cost for promotions could go on and on, and make Forts something that could be usefull and quite fun as well
Well, Just a loose thought-