The Gym Leader Castle in Pokémon Stadium was simple: It was a darkened, curving path leading from spire to spire, with each spire's top in the shape of that Gym Leader's Badge. To advance to the next spire, players would need to defeat each of Kanto's Gym Leaders in turn, eventually making way to the Elite Four and Champion, their rival. After defeating the rival, players will win one rare Pokémon out of a selection of eight, such as one of the Kantostarter Pokémon or Eevee; more than one of which were, at the time, not available without multiple games.
The Gym Leader Castle in Pokémon Stadium 2 was more complex. Instead of it being an actual castle, it featured differently-designed rooms for each of Johto's Gym Leaders, each one representing the leader's type in its design. Like its predecessor, players would need to defeat each of Johto's Gym Leaders in turn, now including an intervention by Team Rocket between the Olivine and Mahogany Gyms, and eventually making their way to the Elite Four and its Champion, Lance. Upon his defeat, if all Pokémon on the player's team came from a Game Boy cartridge, one of them could re-learn a move that was available to be learned by level-up.
After the defeat of Lance, mirroring the story progression in Gold, Silver, and Crystal, players may challenge a revamped version of the Kanto Gym Leader Castle. Though it has the same battle arenas as the first game, the design significantly differs: first, the eight badges of Kanto are on the sides of the castle's spires, and the eight are arranged closely together. Each Gym Leader has no disciples (players battle the leaders immediately) and they can be defeated in any order. After the defeat of Kanto's set of eight, Trainers unlock the chance to battle against Red, who, unlike his Game Boy counterpart, does not have a team that one would see in Pokémon Yellow.
If the Rival is beaten in Round 1, then the prize Pokémon will be holding a Normal Box if traded to Generation II, which contains a Silver Trophy. If the Rival is beaten in Round 2, then the prize Pokémon will be holding a Gorgeous Box instead, which contains a Gold Trophy.
Pokémon Stadium 2
For beating the Gym Leader Castle in Round 1, the player will receive Doduo (if not all Stadium Cups have been beaten in Round 1) or Dodrio (if all Stadium Cups have been beaten in Round 1) Sticker mode for the Game Boy Tower for Generation I games.
For beating the Gym Leader Castle in Round 2, the player will receive Doduo (if not all Stadium Cups have been beaten in Round 2) or Dodrio (if all Stadium Cups have been beaten in Round 2) Sticker mode for the Game Boy Tower for Generation II games.
Trivia
The concept seems to have been abandoned in the same way that the Stadium series itself was, with no Gym Leader Castle for Hoenn, Sinnoh or Unova. However, Mt. Battle may be considered a spiritual successor, as it is divided into zones with Area Leaders.
