Book or Fairy Tale?

Trying to remember either a fantasy book or an actual fairy tale. At the moment I can't remember which it was. Basic plot, as far as I recall, was that there was a door in a castle which was never supposed to be opened. Kid opens it, unleashes goblins that had been sealed there. They seal the kid in their place for a while, though he ends up getting out eventually. Don't remember how. Goblins steal the girl he's in love with (possibly the princess?) and take her to their land, Nilbog. Kid has to go rescue her. Is this ringing any bells for anyone?

Sorry, that's really all I can remember, I think the last time I read this I was about ten or so. If I can find it again, and if it's actually as good as I'm remembering it being, I'm thinking of getting a copy and gifting it to a friend with a kid about the right age for it.

May end up posting this to a book-finding community on LJ but I figured I'd check with my friends first. :)