In your example, I'd question why a character would know that is a door to the elemental plane of Shadow.

Usually my rule of thumb (With any skill) is "Justify it to me" then based on their justification I determine if they can just "roll it away". Also allows for outside the obvious box use. And I personally cringe every time a player goes, "Okay, what knowledge skill do I need to roll to know about..." If you as a player can't figure out if Knowledge (The Planes) or Knowledge (Architecture) applies then how would your character know if he's looking at something extra-planar or just a feat of neat engineering? Try one or the other and give me a reason why you think that. Even if it's wrong it might still reveal stuff.