Go down to the cellar
From the Super Mario Wiki
Go down to the cellar is the fourth story of chapter one in the game Wario Land II. With the masses of water gone, Wario is finally able to enter the basement of his castle. Numerous bat-like creatures lurk the darkness and there are many opportunities to get the flat ability. If he wants to return to his normal form, Wario can just use some of the water that drips from the ceiling, or one of the several grabbing bats.
Hidden treasure: Vase
The hidden treasure of this level, the Vase, is located in the last area of the stage. The player will need to change to Flat Wario in order to reach the location. From the goal door he should climb the platforms visible to the left. He then has to glide through a narrow gap to the left. A bat will grab Wario and restore his true form so he can enter the door.
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