Olivia's Princess practice cake

Olivia is having a princess party for her fourth birthday in two weeks and has requested a castle cake with princesses on it. I've never "built" anything, so I was unsure how to go about making a square castle on top of a round cake with correct deminsions, proportions, and support. I made a practice cake this weekend to make sure I was prepared and had an action plan for the big day. Here are few step-by-step pictures...let me know what you think! P.S. I didn't really decorate it but trying to make sure it was stable and proportionate.