Another idea for decorating a birthday cake. Unfortunately, the only photo I have is old and bad. I tried to fix it in Photoshop and this is what I could achieve given my Photoshop skills.
The bottom part is a big square with a smaller square on top of it. There is a big tower on top of that. All these parts are cut out of sponge cakes. Stack round cookies sandwiched with vanilla cream for the four towers at the corners. Ice-cream cones make perfect roofs. Decorate the cake with pieces of white chocolate and cookies. The castle cake is very high, so make sure you have enough space in the fridge :)