Princess Peach's Castle Dungeon
From the Super Mario Wiki
Princess Peach's Castle Dungeon is an area appears in Mario & Luigi: Partners in Time. It is opened once the bros. and babies defeat Sunnycide and learn the Bros. Ball. Mario, Luigi, Baby Mario and Baby Luigi came here, they found Fawful and followed him, however, only the babies can talk to him seeing that he's in a Pipe that only the babies can access. The babies can trade Beans for Badges at his store. They can come here only by using a Blue Warp Pipe in Peach's Castle. Several Goombas are found in the dungeon, and they can be defeated.
It also appears in the past under the name Peach's Castle Cellar but has a different layout, it plays the same music and the bros and babies battle a Chain Chomp here.
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