After you have 120 Stars, max out your power meter and shoot yourself onto the castle roof. Grab the Wing Cap, then fly back to the cannon and hop inside. Aim as high as possible with the crosshair lined up with the castle tower. While holding Down on the Control Stick, press and hold A. Continue to hold Down and A until you land on the highest platform on the castle tower. Make sure you're at the front side of the castle, and walk straight at the wall. It may take a few tries, but eventually Mario will walk through the wall and drop down inside a strange room. It looks like the door will take you outside the castle, but when you open it, you'll end up inside the castle. Now the door looks like it will take you inside the castle, but instead you end up back in the room. To escape, walk through the wall when you can see the inside of the castle, and you'll end up inside the castle.