The Ruined Hall

From the Super Mario Wiki

The Ruined Hall is a boss stage in the Subspace Emissary mode of Super Smash Bros. Brawl. It is played by the Pokémon Trainer and Lucas. The location is underground, and shown to be very deep and empty. Its actual purpose is unknown. The only objective in this stage is to defeat the boss, Galleom. This boss was first fought by Marth, Meta Knight and Ike above ground, right near the underground Ruined Hall. However, Galleom was not completely destroyed; it jumped and landed directly above this Hall where Lucas and the Trainer has just entered. Because of its immense weight, the ground crumbled to pieces and Galleom fell through to the Hall. It was still capable of fighting, so Lucas and the Trainer had to finish it for good in this empty Hall. After Galleom is destroyed, the Pokémon Trainer and Lucas join the swordsmen, Marth, Meta Knight and Ike.

The Ruined Hall is cylinder shaped like Bianco Hills's Windmill Tower of which Petey is fought in.