On the title screen, hold down 'L SHIFT', 'S', 'U', and 'B'. You will then be presented with a subgame menu. Once you exit this menu the game will start. This also enables the ingame cheatmode, just press 'F10' for 6 more lives.
To see the game ending go to the secret menu (see above) and hold down both shift keys while selecting 'Zak`s tower subgame'.