In the original Game Boy version, entering Left, Right, B, A, Up, Down, A, B, Right, Left while at the Demo screen will show you a developer's message saying:

"PICT OF LOGON PRESENTS YOU THIS OTHER PART OF THE DEMO. THE 3D SCROLL DEMO.
WELCOME TO THE AMSTRAD CHEAT! EVERYBODY WANTS TO BE GREETED IN A GAMEBOY GAME,I KNOW..
GREETINX TO.. ALL CPC FREAX! VERY SPECIAL THANX TO.. BSC AND
KANGAROO MUSIQUE OF HJT. WITHOUT BSC'S SOUNDTRAKKER AND KANGAROO'S COMPOSING ABILITIES
THERE WOULD NOT BE ANY COOL MUSIC IN THIS GAME. DANKE THORSTI UND OLLI!
IF YOU DO NOT KNOW WHAT I AM TALKING ABOUT..
THE AMSTRAD CPC IS AN 8 BIT HOME COMPUTER AND WAS PUBLISHED IN EUROPE BACK IN 1984.
IT IS LIKE A GAMEBOY WITH COLOUR SCREEN AND KEYBOARD.
MEMORY FULL,BYE..."