I remember seeing the movie when I was like 8 or 9, 2008/09, but it could be older than that.
It was about this kid who found himself in another world. I forget if he was taken, teleported, or what. But the King holds a huge parade in his honor. He meets the princess; they're supposed to marry I think. And the kid's shown a really big door he's not supposed to open, and is told very clearly DO NOT open it, and then given the key I think, that or he takes it later. After the parade when everyone is sleeping he goes and opens the door because he's curious about what is inside. And the great big bad comes flooding out and makes a mess of everything. I want to say he was this shadowy embodiment of evil or something, but I can't really remember anything besides the parade, door, princess, king, and that it starred the boy.