NoirResurection

Best answer as chosen by Asker / Ratings
Im very sure you have already done the circle in that level. Heres what you need to do.

You need to go the the level Forest of Illusion 3. You will need to pass th half way marker once you do that there should be a foot ball player that separates into 3. Right behind him is a large green pipe that you need to jump over to get to the other side. There will also be a yoshi coin after the pipe. What you need to do is go into that pipe and there is where you will find the key and the hole to get to the castle.