My soon to be 5 year old daughter came to me one night saying she wanted a castle cake for her pending birthday. After checking this wonderful site I had some great ideas. The next morning the request became a Mermaid Castle cake. A what??? After a bit of thought a sandcastle birthday cake came to mind. I was surprised to find some ideas for that as well on this site.
I settled on a 10x10x2 bottom layer (chose strawberry) and a 6x3 round top layer (dark chocolate). I tried Wilson's buttercream in a box (just add butter and milk) - it was very tasty, not too sweet. Crushed vanilla wafer cookies for covering the frosting of the bottom layer and crushed pecan sandies for covering the frosting of the top layer.
The turrets were made up from cake cones (bottom) which were dipped in melted peanut butter candy wafers then rolled in the left over cookie crumbs and tip here is to put them immediately into the freezer and they set up in minutes. The roofs (or turret tops) were sugar cones dipped in melted pink candy wafers then sprinkled with large clear sugar glitter (cake decorating section of craft store).
As the cake had to travel to the party destination I completely assembled the cake layers and took skewers and pierced through the cake cones and into the cake in the desired locations. I brought the turret tops and just set them on top of the cake cones once we safely arrived at the party destination.
Best part about this cake (besides my daughter loving it) was the cones tasted awesome. The kids loved the tops (pink with glitter) and the adults loved the bottoms.
Could have added some candy shells and fishes, etc. - had every intention but after 5 hours I simply ran out of time. But taking the time to make the cake with my daughter was worth a few missing decorations.