Hoi

 CHEAT MODE:

 Try the following to gain access to the first four levels
 (the last level, level 5, is only available when you complete level 4):
 On the level-access screen, place the "LVL" mouse pointer in the far
 bottom right of the green quarter screen. The bottom "L" must be
 positioned in the corner with pixel perfect precision (two pixels on the
 pointer will be hanging over the edge to the right). Click the left mouse
 button, then shift the "LVL" mouse pointer to the very top left of the
 screen (as far as you can physically move the mouse to the top left) and
 click the left mouse button again. Now you can also get 12 lives by
 pressing <F4> during the game.