From the Super Mario Wiki
The Koopaseum is a large, Colosseum-like structure in the middle of the Gritzy Desert that appears in Mario & Luigi: Partners in Time. In the past, the Mario brothers and the Baby Mario brothers came to the Koopaseum looking for Princess Peach. Once the brothers entered, they saw Peach being fed to Petey Piranha on Princess Shroob's commands.
In the Koopaseum, the brothers fight Shrooboid Brat in the middle of a stage with millions of Shroobs watching them. Even though the brothers defeat Shrooboid Brat, the brothers are taken down by lots of Shroobs with hammers. The adult and baby Mario brothers are then dropped to Gritzy Caves, which is right below the colosseum.
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