How do I get item to open the shock door in the Castle?
The funny symbols on the ground in the castle are teleporters that will warp you around to different points in the castle. One of them will lead you to a hallway where you will find a NPC in purple, who (As far as I can tell) says she has dropped an important item. That item can be found by backtracking towards the teleporter and constantly pressing the action button to search the floor. there is a post earlier in this thread with pretty much the exact position of the item. Once you find this, you enter a fight, and you get the item to open all the doors to the castle.