Fly Castle Window

it was my fav movie from my childhood. about a boy who was in his bedroom and a flying ship came to his window, he got on it and it took him to this land. he met with the king at this castle and he had a daughter who was a princess. she took him around the castle to this door that was never supposed to be opened during a castle ball, they opened it and there was a lot of black/purple goo stuff that was evil and it ended up taking over the place during the castles ball and took the princess hostage. the boy had to figure out a way how to defeat the evil stuff and get the princess back and the land back to normal. does anyone have any idea what this movie may be called? i watched it when i was about 5 so it was out during the early 90s but could have been made earlier. it isnt really a popular movie, but all of my friends said they slightly remember seeing it but cant remember the name. my mom cant remember the name of it either.