Hi, I can't post currently but I can PM your answer, feel free to post it up so others know the answer too.
Fort's and Citadels are not owned as such, like a city or a unit, where by you the player control them, they are a tile improvement, like a farm.
It has already been confirmed that Forts and Citadels don't give a bonus in enemy territory, i.e if an enemy spearman comes into your territory and defeats a unit on a fort, the fort is still just an improvement and will provide no bonus to the spearman. Similarily, if they built one in your territory it would have no effect.
So whomever owns the territory where the fort is built is the only player who gets the defensive bonus, an ally will also get it one would assume.
Neutral territory, (that which has no cities borders on it) would provide the fort's bonus to any unit stationed on it, so if you have your forts built to bolster your defensive line in neutral territory, then yes if this is captured then the enemy gets its benefit, but not on anything in your own borders.