It doesn't seem logical that fully maintained and manned forts simply let invaders to pass by. This is especially frustrating in case of separatist rebels which can cross the border, move to any province they like and siege it, disregarding any fortifications on their path.
See also https://forum.paradoxplaza.com/foru...black-flag-when-in-neutral-territory.1455880/ for alternative approaches to foreign rebels problem (I don't support initial suggestion but there are interesting ideas in comments).
See also https://forum.paradoxplaza.com/foru...black-flag-when-in-neutral-territory.1455880/ for alternative approaches to foreign rebels problem (I don't support initial suggestion but there are interesting ideas in comments).