Attach presentation board. Before decorating, position the printed presentation board around the castle so it fits without any gaps. Secure the board with dots of icing if necessary.
Make front door. Roll out red fondant 1/16 in. thick. Cut out door. Use toothpick to mark slats on door. Attach door to castle. Use white icing to outline door. Cut candies in half and attach around door.
Make windows. Roll out green fondant 1/16 in. thick. Cut out windows, attach to castle. Use black icing to outline windows. Use white icing to pipe lines on windowsills. Attach stars inside windows.
Decorate castle. Attach candy, coin and star sprinkles. Attach Mario and mushrooms icing decorations. Use black icing to outline ramparts. Use white icing to pipe lines of show on top of ramparts. Attach candy.
Outline castle. Use white icing to pipe lines on corners of castle.
Decorate mushroom cookie. Roll out red fondant 1/16 in. thick. Cover top of mushroom cookie. Use white icing to fill in dots. Use black icing to pipe dot eyes.