I see this problem differently, as mainly exhibiting a problem that cannot be analyzed without self-reference. Self-reference causes problems.<p>The more obvious solution that the postscript hints at is the question "what would you say if I asked you whether [this door] leads to the castle?". Here, we immediately cancel out the influence of whatever goblin we're speaking to, and we get the right answer, whereas in the movie's solution we immediately incorporate the influence of the lying goblin and get the wrong answer.<p>But the movie's solution is better for the movie, because "what would you say if I asked you X?" is a normal English way to ask "X", and in this case, where the two questions are different, the audience is guaranteed to be confused.