If you conquer all of the barracks, weakening your opponents' 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 you must conquer the fortress within 10 minutes or all of the barracks' functions will return to normal.
"As a mercenary, what do you..."