A pair of unused text files featuring an abstract of the game's plot and a short list of developer credits can be found in the game's data:

•abstract.txt:

THE APPRENTICE
This is the story of a week in the life of the
wizards apprentice, Marvin. You must successfully
complete the tasks set for you by Gandorph S.
Wandburner. This will take you to many different
places and you will have to solve various puzzles
and defeat numerous enemies to accomplish your tasks.

•bibliographic.txt:

The Apprentice
Programming by Tim Moss
Graphics by Luke Verhulst
Game design by Luke Verhulst and Tim Moss
Music by Joost Egmond

A 1994 Vision Factory production in cooperation with
Philips Interative Media inc.
Contributed by MehDeletingLater
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A clone of Space Invaders, referred to as "Marvinvaders" by the game's developers, can be accessed on the main menu by holding Button 2 and pressing Up/Right/Down/Left/Up/Down/Left/Right. A reskin of this game would appear in a later Vision Factory title, "Golden Oldies I".

Of note about the game's HUD is that "裸猫女" translates to "Naked Cat Woman".
Contributed by MehDeletingLater
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On the main menu, hold Button 2 and press Up/Up/Down/Down/Right/Right/Left/Left to bring up a cheat menu that contains a level select, a life counter changer, and an invincibility toggle.
Contributed by MehDeletingLater
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NSFW - This trivia is considered "Not Safe for Work" - Click to Reveal
There are eight hidden Game Over animations which can be accessed through the use of cheat codes. They include a "Fatality" animation, two different "Animality" animations and five "Nudality" animations. The codes were first revealed by one of the developers under the alias "cdidev", in a forum post on April 24, 2005, approximately 11 years after the game's release. These Game Over animations are obvious parodies of the "Fatality" system in the Mortal Kombat series.
Contributed by KnowledgeBase