When you are at the top of the castle talk to Yoshi and all now Walk along the castle (And if you remember how there is like a entrance to the outside(Back)of the castle in the castle the basement all the Gosts are)well walk to the end of the top of the castle and look down and you Will not see the back of the castle but instead see an Huge Lake now Here it goes:Stop looking down and press the right C-Button a few times and you will SEE THROUGH THE WHOLE CASTLE!!!!