That door looks like this:
http://www.neopets.c...en/nox_machine/
Notice how you're "in Fort Nox"? Wouldn't it makes sense that a door that was actually there would also be represented on the board?
And wouldn't it also make sense that you wouldn't be able to open it in the game because you need the NC keys or whatever?
I believe you're right. That door is only accessible to NC players.