From Bulbapedia, the community-driven Pokémon encyclopedia.
Pummelo Stadium (Japanese: カンキツスタジアム Kankitsu Stadium) is the main landmark of Pummelo Island. The leader of Pummelo Stadium is Drake, the leader of the Orange Crew. All four Orange League Gym Badges are required to challenge him. Trainers who defeat him in a Full Battle receive the Winner's Trophy and are accepted into the Hall of Fame, effectively becoming Pokémon League Champions.
In the anime
Pummelo Stadium appeared in three episodes of the anime: A Shipful of Shivers, Hello, Pummelo! and Enter the Dragonite.
The stadium sits atop a hill overlooking the city. A statue of a Dragonite is situated at the top of the stadium. It is a Colosseum with lights and a scoreboard, its size rivaling even Indigo Plateau. In the Winner's Cup Competition, whenever three of either Trainer's Pokémon are unable to battle, the field is changed.
Pummelo Stadium first appeared in a flashback in A Shipful of Shivers, when a Haunter created an illusion of its Trainer, the Captain, winning the championship three hundred years before, using his Gastly against another Trainer's Beedrill.
Ash challenged Drake for a championship battle on his journey through the Orange Islands. Ash competed on a rock and water field and then a sand field. The long match went to Ash, and he and his Pokémon were honored in the Palace of Victory. It wouldn't seem that Ash had any obligation to take over as the Head Leader and defend the title against challengers as Drake does, however. After accepting the Winner's Trophy, he continued on to the Johto region to compete there.