I have not tried this yet, but I'm going to see if it works when I'm playing tonight:

Your army's zone of control is fairly small, but an enemy can't just move through it unless he wants to fight you first, right? So if you want to block a road in a mountain pass with forts, perhaps two forts side-by-side would work. It would be a matter of extending your zone of control completely across the pass so that the enemy could not technically move through it.