Festung Luxemburg
playing modded but this is 100% vanilla, got an event to instantly upgrade my forts as they were already being upgraded but because of the non-stop sieging of this province the fort was delayed
the moment it was built, I triggered the bock mission which turned into a lvl6 fort but then I guess the lvl4 event finally had an effect on that province
wonder if this is known by the community, that you can stack forts