Step 1: Bake a 13x9 inch cake (whether you use cake mix or make it from scratch!). When the cake is cool, remove it from the pan and cut in half crosswise, into 2 rectangles. Cut one of the halves into 3 equal pieces lengthwise. Cut one of those pieces in half again to make 2 small squares. Round off the end of each of those small squares. (See the diagram.)
Step 2: Put rectangle A on your base (I used a tray covered with foil). Frost the top. Then place pieces B and C on top of piece A, at the edges (see the diagram below).
TIP: (I learned this the hard way) Use a thin layer of frosting over the whole cake, especially the cut sides of the cake, before putting on the final layer of frosting. This will help seal in the crumbs so they won't show on the final layer.
Step 3: Place the D pieces at the front corners of the cake so that they extend a bit beyond piece A. These are the towers. Frost them with a thin layer. Tint remaining frosting whatever color you want and lavishly frost the entire cake. A buttercream frosting works well.
Step 5: Use pieces of chocolate bar to make windows and the castle door. For battlements around the cake use pillow mints, or gumdrops, or any other kind of square or mounded candy/sweet you can find.
Step 6: Enjoy! Your cake is designed, decorated and ready to eat!