Heart Gold and Soul Silver: Battle Tower
The Battle Tower is located on the north end of the Battle Frontier. Talk to the worker in the middle to enter a Single or Double Battle challenge. Talk to the worker on the left to enter a Multi Battle with a friend wirelessly. Talk to the worker on the right to battle other teams of a Wi-Fi connection.
The Battle Tower is very straightforward. You simply battle other trainers one by one with no extra frills. You are awarded 3 Battle Points after you win your seventh battle.
Talking to the right worker at the desk will allow you to battle over the Nintendo Wi-Fi connection. You won't actually battle other people, but instead you will battle other people's Pokemon which are computer-controlled. When you connect, your team will be saved for other people to battle against.