I´ve never used forts so far, since I didn´t know they keep rebels down. Otherwise, what´s the point? I can place an army in the open field - or preferably on a bridge - just as well, since I don´t think that it makes any difference of whether the army just stands about in the landscape or camps in a fort, and it can´t be besieged (losing men each turn without any gain - besieging units don´t lose men) or get trapped within the fort in battle.