I started out really ambitious, and probably would have done better if I'd allowed for more time (story of my life). The beautiful thing about this castle cake was that it was actually relatively easy - just a couple of square layers, a couple of round layers, and cupcakes.
You'll need: 3 boxed cakes or homemade equivalent 8x8 square, 8" round, and cupcake pans lots of icing! fondant in the colors of your choosing, or buy white and use food coloring. fondant cutters or cookie cutters of your choosing. An 8" cardboard cake round 6 straws sugar cones toothpicks paper flags (I cut 'em out of construction paper) scotch tape A giant white chocolate bar Twizzlers pull-n-peel or just Twizzlers if you like (for drawbridge)
Main part of cake - the castle: First I baked all of the cake - I cheated and used white and confetti boxed cakes, making my life much easier! It took about 3 boxes, one for the two square layers, one for the two round layers, and one for the cupcakes, of which we had extras. Just bake according to directions on box.
Stack the two square layers with a bit of icing in between (I used homemade buttercream but you can use what you like, including store bought) for stability. I also iced the top, but that's not necessary - I just like homemade buttercream!
Place 4 of the straws into the cake, in the center piercing both layers, where you will be stacking the round cakes. Try to place them equidistant as they will serve as support for the round cakes. Cut the straws flush or just above the top of the stacked square cakes.
Place the first round cake layer on the 8" cardboard round. Ice the top and stack on the second layer, icing it as well. Place entire round cake, with cardboard, on the center of the stacked square cakes, using the straws to help support the weight of the round layers.
Cover entire 4 layers with fondant and decorate as you choose. I used colored fondant (I also added a bit of almond extract to my fondant for flavor) and cut it out into fun shapes. Very lightly wet the back of the shapes, and "glue" them onto the cake as you choose. Be careful, as if there's too much water, the colors will run onto the white fondant.
To make drawbridge: Cut the chocolate bar into two equal sized pieces. Cut off corners of both pieces to make the open door and the opening. Use a little bit of icing as glue and place 1 piece on cake for the door opening. Place other piece directly in front of it on whatever plate/pedestal you are using to be the door, lying open.
Peel two pieces of Twizzlers and pipe some icing to glue it in place on each of the four corners created by the door and opening.
To make turrets: Here's where the cupcakes come in. I used 3 stacked cupcakes (6 total), iced in between, and held together with a straw through the center of all 3 for support. Then I wrapped them, all together, in fondant. Turn a sugar cone upside down on top, and pipe some icing around it to hold it in place. Tape your flags to your toothpicks, and stick the toothpicks into the top of the cones, stabilizing with more piped icing.
Entirely ice an additional cupcake, place on top center of cake, piping decorative icing around the base of the cupcake. Repeat the sugar cone/flag directions for the top of the castle.