Type in "KRIS" on the option screen and you will then be able to
        skip through the levels with the 'F1' key.

        When you collect the pod that produces four bases, placed at equal
        intervals along the bottom of the screen, don't just leave them
        there. If the invaders are quite low, shoot the bases up the screen
        and into the alien swarms. This way you are guaranteed of killing
        aliens as they die upon contact with bases, rather than allowing the
        space fiends to destroy them before they come down.