My daughter is very into princesses and castles, so we had to have a princess castle cake for her 3rd birthday.
I started with 2 rectangular cakes, one was 9x13 and the other was 8x11. I evened off the rounded tops, then cut the larger one into 2 larger squares, and the smaller one into 2 smaller squares.
I stacked the larger squares, then the 2 smaller squares on top, with frosting between all the layers. I cut off the corners of the lower portion of the cake so the towers could be added later.
I frosted all the layers at this point. I had trouble getting the frosting to stick at first, and there were lots of crumbs where the cake had been cut. To fix this, after I had put on a thin layer of frosting I put the cake in the fridge for a couple of hours. When I took it out again I was able to add a second layer of frosting and it looked fine (whew!).
I made each tower out of one cake cone on the bottom and one sugar cone on the top. To give the cake cones some more stability for the base of the towers, I filled them halfway with cake batter and baked them (just set in a muffin tin and bake for 15-20 minutes). I frosted each cone and turned the sugar cone upside down inside the top of the cake cone. They fit great. I piped extra frosting around to cover the seams.
I also put one sugar cone upside down on the top of the cake for a central turret. I stuck toothpicks with flags in the tip of each sugar cone. The top one, which got cut off in the picture, says "Aidan Rose is 3."
I wanted a very pastel, girly look, so I used colored mini-marshmallows to finish off the top of the castle, and piped sparkly colored icing in squiggles down the sides of each tower. I also put a row of marshmallows around the base of the castle and around each turret (make sure to do this before the frosting dries or they won't stick!).
The door of the castle is made of graham cracker sticks, and the drawbridge is Oreo sticks with peel-apart Twizzlers for chains. I tinted some of the leftover frosting blue and smeared it around the base for a moat.
The little girls at the party thought the cake was the most amazing thing they had ever seen, and the moms were pretty impressed too. Even I was amazed at how well it turned out, considering how it looked before I added the second coat of frosting!