From the Super Mario Wiki, the Mario encyclopedia
To access this room, Luigi must use the Dark-Light Device in the Cage-Lift Room to reveal a coffin that will be able to bring him to the Waterwheel Room. There is a banner on the west wall, and a bench with some soda in the northwest corner. There is a little staircase on the south side of the room which leads to a little pathway filled with water, there is also a paper boat floating in that water. There is also a wooden platform with three hooded figures holding crossbows, if Luigi tries to cross the platform the figures will start to bombard Luigi with arrows. To cross the platform Luigi must pull the rope next to the first figure which causes a part of the platform above the figures to lean. Luigi must then send Gooigi across the platform because the arrows don't hurt him. Once Gooigi is across there is another rope on the platform above the third figure, Gooigi must use the Poltergust G-00 to get this rope. Gooigi must then pull the rope to cause the entire platform to fall in front of the figures. Now Luigi can cross without getting hit by arrows. On the other side is a little platform with a treasure chest on it. Inside the treasure chest is some money and the key to the Dungeon.
There are a few coins and Gold Bars on the shelf in the southeast corner. There are also a bunch of coins at the end of the little pathway filled with water. To get the coins Luigi must use the Poltergust G-00.
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