The Castle (1989) - meta game

HIGHLY recommended to watch this video before doing anything else, because there I explain everything:

Too long; didn't watch: I have found a very weird Easter egg or even more weird bug in old "The Castle" 1989 by accident. Shortly: the whole game plot is around fighting with unknown monster called The Entity hiding below the castle to save the castle. But if you open the game for the first time (or delete licension file) so it asks you to enter activation key once again, and instead of it enter "entity", game unexpectedly starts in 3D-interface (the whole game was 100% 2D! and the game is y. 1989 game boy!!)

And there happens some interesting shit, to I did convert and publish the file of the game here if someone would be interested just like me.

If you have any ideas or find something else, please write in the comments. Also if someone has the same game, PLEASE try to enter "entity" and write to me if you got the same result so I could know if it is just my version corrupted/bugging or it was an easter egg.

p.s. You can cut the paintings in the room 3!!!! You just need to go REALLY close to the paintings!!

p.s.s. Found some interesting files in the core file, uploading also for a small price (for same reason: i spent money on conversion program and will VERY appreciate any donation!)

Publishing also some scans (taken from internet because i dont have a scanner; as you can  also notice, they are for commodoro cartirdge):

(thanks TheWiredOS for scans)