Which Pokémon game first featured the Battle Frontier?

Pokémon Emerald was the first Pokémon game to feature the Battle Frontier.

Released for the Game Boy Advance in 2004, Emerald is an enhanced version of Pokémon Ruby and Sapphire. After completing much of the Hoenn adventure, players can visit the Battle Frontier, a large post-game facility built around specialized battle challenges.

The original Battle Frontier contains seven facilities: the Battle Tower, Battle Arena, Battle Factory, Battle Dome, Battle Pike, Battle Palace and Battle Pyramid. Each uses different rules, making the area more varied than ordinary Gym or Pokémon League battles.

Ruby and Sapphire included the Battle Tower but not the full Frontier. Later games reused the name or introduced selected Frontier-style facilities, which can cause confusion when comparing post-game content. Pokémon Platinum later added its own Battle Frontier in the Sinnoh region, but Emerald was the title that established the concept.