From Bulbapedia, the community-driven Pokémon encyclopedia.
The Battle Tower of Hoenn is located south of Route 130 in Pokémon Ruby and Sapphire and on the north side of the Battle Frontier in Pokémon Emerald. In both, its functions are carried over from Pokémon Crystal, though it includes possibilities for double battles and linked double battles in all three GBA games, and wirelessly-linked double battles in Emerald.
Using the same rules and regulations as Crystal's Battle Tower, the single battle version of the Battle Tower features seven trainers in a row with three Pokémon each, with the player taking three of his or her own Pokémon to do battle. Pokémon may be either at level 50, where all opponents will have level 50 Pokémon, or any level, where all opponents' Pokémon will be at the level of the player's highest-level Pokémon. No two Pokémon can have the same item on the same team.
Similar in rules to the single battles, however, players and opponents each have four Pokémon instead of three, and every one of the seven battles is a double battle.
True double battles
Like the regular double battles, this features seven double battles in a row. However, this time, they can be tackled with a computer-controlled opponent. Only two of the player's Pokémon are allowed to compete in this, however, all of the other standard rules apply.
True wireless double battles
Available only in Pokémon Emerald, this allows the player and a friend to fight another pair of friends together. All normal rules apply.
In Pokémon Diamond and Pearl a women in Sinnoh's Battle Tower mentions that she is from Hoenn where another (this) Battle Tower presides.