I think that you MUST put fey NPC in the fort so they guard it against VERY annoying need to run and check fort just to find one low lv player caped it and run.
You can even make a capturing a fort more interesting with 3 capture points that need to be caped before fort change owner and put them outside so enemy cannot get in fort until they are captured.
Every capture point can have commander and 2 guards, after they are captured enemy can get in fort and kill main captain of the fort and from that moment they will have its own NPC garnison and all benefits of fort!