If you conquer all of the barracks, weakening your oppenents' defensive system, the command gate will open. Entering the command post, you will find a Flag of Combat on the first floor that symbolizes the authority of the fort. You can end the battle by hanging it from the flag pole on the very highest floor.
Be warned, though, that all of the barracks' functions will returm to normal after a short time, so you must conquer the fortress withing 10 minutes.
"As a mercenary, what do you...?".