That opens the black treasury, iirc. It's in the castle in the sky, near the puppet king boss.
it worked the next time i tried it ! weird ... im now stuck in the section after the grinding wheel room where there are triggers to push ... ive got 2 of them but the last is blocked by a blue block and theres a bouncing red thing close by that i cant do anything with to interact ... tried the boomerang to hit the trigger but no luck , been all around the stage but cant find anything to help ... any hints ?
really wish there was a walkthrough for this game !
cheers