I think forts are very usefull in certain games even before BtS (in BTS they are very very good)
In my last game on normal i was on a continent with Mali above me near the flaciers, and Rome below me, but through a 2 tile choke point.
There was a Hill with Jungle on it, the BEST part about a fort is you dont need to clear the tile, so the tile was giving +100 defence, add that to Hill Defence Promotions and defence units become almost invincible.
When war finally broke out the romans would try come through the choke point, and i would be forced to attack me tile (even though they could og around it) coz i was bombarding their stack with catapults.
CONCLUSION:
Forts kick ass when you need to defend a choke point