The Battle Tower is a new feature introduced in Pokémon Crystal, located in Olivine City. It's a challenging facility where you can test your Pokémon's strength against a series of trainers in consecutive battles. The goal is to achieve the longest winning streak possible. There are different battle modes, including Single Battle and Double Battle, each with varying rules and restrictions.
To access the Battle Tower, you must first reach Olivine City. You'll need to have a strong, well-balanced team of three Pokémon. The levels of opposing Pokémon will scale with your own, making it a constant challenge regardless of your progress in the main story. Winning streaks earn you Battle Points (BP), which can be exchanged for rare and powerful items. This is an excellent way to train and earn rewards outside of the main story.