Pokemon Crystal also introduced a feature into the Pokémon series which has returned countless times, the Battle Tower.

Battle Tower

Location: Route 42

Located above Route 42 is the first iteraton of the Battle Tower. In the Japanese version, this unlocked with the use of the mobile Adapter and allowed for battle with people across Japan. It also tracked various people, the amount you battled and other such statistics

10 floors: Level 10, 20, 30, 40, 50, 60, 70, 80, 90, 100
Pokémon: Cannot use Mewtwo, Mew, Lugia, Ho-Oh, Celebi
Cannot use two of the same Pokémon or hold item.

Within the English release, this kept the restrictions and levels, but included more AI trainers having you face up against 7 per floor. These trainers used Pokémon from a random selection and you were given Protein, Calcium, Carbos, Iron and Zinc whenever you defeated all seven trainers on the floor.