Board Thread:Found a new Item!/@comment-31450036-20170305034240/@comment-173.248.7.239-20170305211049

That is one of the Endgame fortresses, I have not yet discovered how to activate it, but the portucullis at the entrance is supposed to slam shut when its summoned and then you need to fight it while in the tower.

Again, I personally have no idea how you summon it, but im sure there is a lorebook about it somewhere.