First, head straight south two screens from the brown Basement, past the spike-crusher room and into the room beneath it. Solve the block puzzle to move a block to the lower right. Then go back up two screens, go east, use the conveyor song twice to make your way to the lower right of the screen, and head south. I think there's a key after that stretch of dungeon.
Yes, just found this key… Now what?
Also is it possible to hold more than 999 rupees? Just found the hidden shop.




