This level is a haunted mansion filled with ghosts and tricky platforming. You'll encounter more Boos, as well as Dry Bones (Skeletons) that reassemble after being stomped. The level features several rooms connected by doors. Some doors are fake and will lead you back to an earlier part of the level, so pay attention to which doors you enter. There's a section with falling chandeliers that you need to avoid. A Superball Flower is highly recommended here for dealing with the Dry Bones and other enemies from a distance. Look for it in a hidden block near the beginning of the level.
Puzzle: To progress through the mansion, you often need to find the correct door. If you enter a door and it leads to a dead end or loops you back, try another one. There's usually a visual cue or a specific path to follow.