Actually, Forts are useful. Just not inside your fat cross generally, unless you have a dud desert tile or something.
Especially on maps with lots of mountains, you can often get a tile that the enemy either has to go thru, or spend another dozen turns walking around to another route. Stick it on a hill and plop a couple guerilla promoted archers in it and they will chew up whole stacks.
Naval invasion. Since the AI likes to land its troops in the same spot with every wave, once you know where that spot is, its often worth it to sacrifice a couple workable coast tiles to build forts and stick troops in them. The AI will spend troops on a hopeless amphib assault against fortified troops inside a fort rather than move a few tiles down the coast.