Enable the LEVEL SELECT code that is mentioned above. Go back into level 1, and beat level 1. After beating level 1, the "GAME COMPLETED" message will appear.
Switch the Left Difficulty switch to the left while you're playing the game to show the other towers. Switch it back to the right to choose the tower that is currently displayed.
Use the Level Select code to choose the tower before the one you wish to have infinite lives in. Once in that tower, lose all of your lives. After this move the left difficulty switch to the left. This should be done after you lose your last life, but before the game over screen. The next tower should then appear. Move the switch back to the right for unlimited lives.