The Mystery of Xak

[Nemo]

I'm not sure how many people have played through this game, but last year when I completed it I encountered a bit of a mystery.  In Xak II, in a big castle later in the game, there's a big door on the lobby level that I couldn't figure out how to open.  Also in the same castle there is a part where you can go underwater if you have the scuba gear, I continued to follow the path above and below ground till I reached yet another door with no apparent opening.  As I mentioned, I beat the game, so both doors appear to have no significant importance on the game, yet I'm still interested in knowing what lies beyond these doors.  Anyone ever transgressed them, or even know what the heck I'm talking about.