I have been playing around with the movement rules (from region to region) and occupied structures.
The rules work such that you can not move from a region with an enemy occupied structure to a region you do not control. This is perhaps reasonable, since this prevents armies from bypassing enemy controlled structures such that they would put the enemy in their rear. Thus, if you attempt to march through a region with an occuppied fort to a region you do not have control of, your march will automatically be halted.
However, there is one case where I think the rules seem somewhat odd about this. If you set your posture to assault and you eliminate the enemy garrison on Day #1 and thus take control of the fort/region; your march to another region which you do not control is still halted. In short, you can not fight your way past an enemy fort in a single turn. A weak single unit army inside a fort is enough to bring your giant stack to a halt.
Now, I suspect this is because the rule checks are not applied dynamically during the turn, but only at the start of the turn. (maybe something more should be done to handle this particular case)