Overall, on border, because then when you conquer adjacent province, it's still under fort control. That being said, new mechanics mean that if a fort is in a mountains, you can park an army on it and the ennemy will have to attack you in the mountain, instead of just avoiding your army as it would use to. So depends on terrain, too.
I think they mentionned that France can afford 4 forts at start though, so you probably shouldn't expect to be able to entirely close your border until mid game though.
I think they mentionned that France can afford 4 forts at start though, so you probably shouldn't expect to be able to entirely close your border until mid game though.