How would you walk around the fort? They weren't build in remote location, but on the main routes, to protect them. And, as battle is not a single battle but an abstraction of all battles happening in the province, fort is not a single building but a whole fortification system. It is literally blocking the road.
I think it's the best addition in this patch.
Blocking one Province's road is fine, but blocking every Province around it as well is ridiculous.
If you want to make an impassable wall on your border, it should be seriously expensive... it should involve building forts in all the provinces. Too expensive? Doesn't matter. Either you can pay for that luxury or you can't.
There should also not be this breaching requirement to assault, either. There ought to be reason to want to keep funding forts in peacetime. You know, so that you don't get stomped by assaults constantly.
What I'm afraid of is that warfare will now become a matter of being channeled through your target's provinces (or channeling through their's), with hardly any maneuvering involved. The dance of armies that leads up to battle will be much more predictable, and probably favor whoever's got the larger force.