To get the first bonus level find the area which has 5 small "towers" -- 4 blue surrounding a central red one--just before the level takes a 90-degree turn upward. First shoot the 4 blue ones, then the red one last. The secret path will open up, you will have a different path, you will skip that level's boss, and you will then play the1st bonus level which has a different boss. You also get a 10,000,000 point bonus for getting the different path.