Which Pokémon game introduced the Battle Frontier?

Pokémon Emerald introduced the Battle Frontier.

Released for the Game Boy Advance in Japan in 2004 and internationally in 2005, Pokémon Emerald is an enhanced version of Pokémon Ruby and Sapphire. It retains the Hoenn region but adds a revised storyline involving both Team Magma and Team Aqua, with Rayquaza playing a central role.

The Battle Frontier is a large post-game facility where players face specialized challenges rather than progressing through the normal Gym route. Its seven facilities include the Battle Tower, Battle Dome, Battle Factory, Battle Arena, Battle Pike, Battle Palace, and Battle Pyramid.

The Battle Frontier is often confused with the Battle Tower, which appeared in earlier Pokémon games. The Battle Tower is one facility within Emerald's larger Frontier complex. Later games reused or adapted parts of the Battle Frontier concept, but Emerald was the first main-series title to present the full seven-facility version.