Use a long serrated knife to trim the top of the round cake flat, if needed. Cut a 5 1/2-inch and a 3 1/2-inch circle from the square cake, then stack all three pieces, spreading vanilla frosting between the layers. Frost the stack.

Use a rolling pin or food processor to crumble the wafers. With your hands, press the crumbs into the frosting, completely coating the cake. Spread frosting on the bottom of each ice-cream cup, then set them around the cake. With a serrated knife, carefully trim 1 1/2 inches from the top of 2 of the sugar cones. Set all 6 cones into the cups to complete the turrets, spreading a bit of frosting around the rims to hold them if needed.

Press 2 graham cracker-stick doors into the middle cake layer and rim the rest of the cake with graham cracker-stick halves. Sprinkle the remaining crumbs around the base and add a scattering of candy seashells and rocks. Serves 12.