Fortress Prison

I think that yes these are working, but it depends on whether you sign a contract with the castle the fort is attached to or not.  You must refuse the contract.  Not all forts have a dungeon, only the large ones.  Every member of your party must take the quest before or they won't be able to enter.

if the previous owner of the fort (if it had one) signed the contract, you must wait for it to reset even if you have just taken it over.  Unfortunately, I don't know the timer for this.