Glass Float is a castle made of glass. A fragile glass bridge links it to land, since it is floating offshore. The door can be smashed open, but that would cause the castle to shatter. It must be opened carefully.
Glass Float is made entirely of glass. Bloopers and Scoombas are seen through the floor, but attacking them would cause the floor to break, resulting in a leak. You can safely walk through the castle without being damaged. You will then encounter what appears to be an Iron Goomba.
It's a Trap?!?
The Iron Goomba is not really an Iron Goomba. This is suspected when Shadow Mario's Shadow Balls have no effect. Bowser Jr. can use his Magic Paintbrush to reveal that the Iron Goomba is just a glass statue covered in Disguise Paint. Past the "Iron" Goomba is a stone statue of Gooper Blooper. When the cork is struck, the tentacles will smash the glass floor. The wall then breaks to reveal the Queen Anne's Return.