Referring to the photo, use a toothpick to prick the outline of the door and 2 windows on the frosting. The door is 2 inches wide and 3 inches tall; the windows are 7/8 inch wide and 1 3/4 inches tall (you can make paper patterns for them first if you like). Fit a third decorating bag with another #3 round tip (or wash and dry the first one); add the purple frosting to the bag. Pipe a line of frosting along each pricked outline. Then pipe more frosting inside each shape, spreading to fill with a clean spatula. Exchange the round tip for the basketweave tip, and cover the door with vertical planks, drawing each from bottom to top. Switching back to the round tip, pipe beads around the door: For each, use even pressure to squeeze out a small amount of frosting, then release the pressure and lift the tip. Pipe a vertical line in the center of the door and pipe 2 beads for doorknobs. Also pipe a line for each arched window frame and beads for each windowsill. Affix a small candy heart to each window with a dab of frosting. Fit the decorating bag holding pink frosting with a #12 round tip. Pipe a bead border around the bottom of the cake; pipe another bead border right above the first. Change to a #30 star tip. Referring to the photo, pipe 3 conical towers on top of the cake: For each, first pipe a circle on the cake top and then, without stopping, lift the tip and pipe a spiral of decreasing rounds until the cone is about 2 1/2 inches tall. Stop the pressure and end the cone with a peak. Place a 1-inch sugar heart in the top of each tower. Using a dab of frosting, affix each remaining 1-inch sugar heart between the door and a window. Add the banner and candles if you wish.
Place windows all around the castle so it looks pretty from any angle. Buy enough candy hearts in both sizes to complete the effect.
Excerpted from Cakes for Kids by Matthew Mead. © 2008 Chronicle Books