I have found boss doors are locked that require the Carpenter's Key, the Millionaire's Key, and the Celestial Key, and maybe there are others. How and where can I find all of these keys, and any others I don't know about?
Most of the keys are late game items, so if you're not past 60-80% castle exploration, these will probably contain spoilers
- Celeste's Key - Hall of Termination
- When you exit the teleporter room, head straight up and then open a secret room in the ceiling
- Carpenter's Key - Dian Cécht Cathedral
- At the top of the right-most spire
- Requires the Invert shard
- Millionaire's Key - Behemoth's Den
- From the entrance head left, then down, then right. Near the top of this room is a chest on a ledge that you need Invert or High Jump to reach
- Warhorse's Key - Glacial Tomb
- From the teleporter, head as far left as you can. A Gusion Cannon is obscuring the chest in the left-most room.