To get the Secret Star in Castle, stand on the sun platform in the middle of the main room in the castle with Mario. Gaze up at the sun above the final Boss door. You must perform this trick with Mario because he is the only person that can fly. Inside, you must collect eight red coins and a Star will appear on the center platform. Note - You can only receive this star after you reach a certain point in the game. You will know you have reached this point when the sun shines through onto the platform.