I'm not sure if you can just go upto the top of the castle. I have never heard of this being done before. You can eat in the castle and thats a lovely experiance. They have character dining with the princessess and we loved it. There is a great shop under the castle in the walkthough bit. It sells some great art, we got a lovely picture of Miss Piggy last year for my Mil and she loved it.
This year disney have a new theme, when the 50th celebrations end a new celebration is starting called the years of dreams. One of the dreams you can have come true is to stay overnight in the castle! I have also read though that the whole thing is only open to US residents, don't know if thats true.