I've checked ingame on some of the things I wasn't sure about, and fortresses don't have any of the neat things it had in previous Civs, only +25% def bonus, nothing else, no benefits of any kind are stated in the Civilopedia (in Civ 3 I think they also ended an enemies turn in they entered it, basically making it a high movement penalty square to enemies as well).
The AI is quite smart about its usage of the current Forts (they intelligently don't bother making them

), I can see how some of the ideas would definately require some AI changes (worrying about adjacency moves and the like). Still it'd be nice to build useful forts.