Instructions
Defeat all of the gym masters in the game as well as the Elite 4. You need to complete the game before you can enter Bell Tower, because the tower guard requires all eight gym badges. You should already have the Rainbow Wing from the Radio Tower, because defeating Team Rocket at the Radio Tower is necessary to complete the game.
Travel to the Dance Hall in Ecruteak City. You must battle the five Kimono Girls consecutively. If you defeat all five, you receive the Clear Bell.
Travel to the entrance of the Bell Tower in Ecruteak City.
Speak to the monk guarding the Bell Tower. Show him the eight gym badges, Rainbow Wing and Clear Bell. He allows you to pass.
Climb the series of ladders until you reach the entrance at the top of the tower. Go through the door to enter the tower.