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 eaten by Petey Piranha, by 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 Shroobswith hammers. The adult and baby Mario brothers are then dropped to Gritzy Caves, which is right below the colosseum.
It is unknown who really owns the colosseum but because of its name, it is very possible it was once owned by the Koopas, before being taken over by the Shroobs. It is also possible that when the Shroobs came to Gritzy Desert, the Koopaseum was already abandoned.