My son, turning six, had a knight-themed birthday party at the Higgins Armory Museum so I made him a Homemade Knights and Dragons Castle Birthday Cake. I wanted it to be worthy of knights and dragons and not resemble a princess castle cake, so I made it with grey rectangular walls and four large square towers.
This was a large cake made with two stacked 18" x 12" dense from-scratch cakes. I cut out the middle to form the four towers. I made sure to support each tower with cardboard set on wooden dowels cut to the right height, and then I put a pointed bamboo skewer down through all the cake and the cardboard to keep it from sliding. (I've had issues in the past.) The ramparts were formed with cut up rice crispy treats and the arch with a graham cracker, it was all frosted grey and then I scratched in a brick pattern using a ruler for the horizontal lines and a toothpick for the vertical lines.
I *lightly* sprayed it with black spray color to add some interest to the color. I piped on white window borders with a #4 tip and then painted in black gel frosting that you buy in a tube from the store. (I didn't want to pipe it in because it might get too thick and drippy.)
Then I placed the cake (on a cardboard cake board) onto a large covered wooden cake board, added a chocolate frosted graham cracker for a drawbridge, piped on blue frosting in swirls around the castle with a star tip for a moat, and spread green frosting out in front of the castle for the grounds. I added a dragon, knight, and catapult figures that we purchased from the toy store.
The catapult was launching "rocks" made from cake balls (cake mushed with frosting and formed into balls.) One of the "rocks" is impaled into the castle wall with cracks. It was fun building this castle out of cake and all the kids loved it.