Inufuto is a japanese who write many games, each time for several platforms including Amstrad CPC. His last game is Mieyen. You will find there the Youtube channel of Inufuto.

You must defeat all the invisible monsters to level up. Watch the crumbs that the monsters eat disappear, so that you can guess their location and fire at them. Sometimes, the monsters show up for a brief instant. You will fail if all the hamburgers are taken, or if you touch a monster.