I saw it when I was little (sometime in the 90s) and I don't remember much of it very clearly. It's an animated children's movie, and I remember something about some kind of black liquid (or something) taking over a castle (at least I think it was a castle). I'm also pretty sure the main character was a boy, and I remember SOMETHING about a key (but I can't remember if he had to find it or what the significance of it was or anything else). I think at the end the boy is in a hot air balloon, but I might be confusing that with another movie.

I know this is incredibly vague, but ANYTHING might help.

THANKS!

Little Nemo: Adventures in Slumberland (http://www.imdb.com/title/tt0104740/)