I noticed that some people here build fortresses on every tile along the border with AI.
Does it really make any sense??
Hence, when you start war what was your border will become a middle of your empire and it's just annoying to have a line of those fortresses on the map...
Personally, I almost never build any fortresses with exceptions of chokepoints maybe... It's always better just to have border cities with few defensive units and also some Horsemen/Knights/Cavalry/Tanks in them for defensive counterattack.
Military Units have much better use in cities especially if your goverment can make use of Military Police. Plus, you have to have a lot more units to keep in fortresses which can mean more money spend on defensive unit support than otherwise might be required