IVANHOE

Press P for pause then type ZOMBINETTE and unpause.
Press N to skip levels.

If that doesn't work try the following

Pause the game and type 'JC IS THE BEST'.
Unpause the game and the these in-game keys will be enabled:

      [N] - Next level.
 [Delete] - Kill all on screen baddies.
   [Ctrl] - Kill the big boss in the bonus stage.