I worked my way through the House of Whatsit (I can’t remember it’s real name) and found three buttons which seem to activate locks. Or parts of them, anyway. I then got to a door that I need a Yellow Dragon Key to open, and generally ran out of places to go. I warped back to the shop, and bought some stuff, before going back into the castle and trying the Anti-Soul Mysteries “level”. No, really. That’s what it’s called.
It seems a lot easier than the other bit, so perhaps you’re supposed to do this first? I made it up to the third floor, but I’m stuck needing a key again. I did, however, find the Yellow Dragon Key, so all wasn’t lost. I’ve found what I think is the boss too, albeit shattered into bits. There’s a giant Polo on the wall with a word on it, that I need to find an ‘E’ for, I think.
Anyway, I warped out, bought stuff, and found a save point.