Pipe icing to outline window shapes on cake; top each window with 1 banana-shaped candy. Push 1 candy wafer, flat side up, into cake at bottom edge of each window to look like windowsill. Pipe icing to outline door, roof and entrance of castle. Place mints in icing at entrance of castle to look like walkway. Add gum to rooftop for shingles. Top each tower with candle. Store loosely covered.