James, it's starting out Great. i'd suggest not making it so narrow, especially on the side away from the steps. i also suggest you build this with something like Magic-Sculpt epoxi clay (magicsculp.com) You can build a strong and light weight structure, even with thin walls. you could even build over your cardboard structure, and once the clay cures you can completely remove the cardboard form. i'm happy to help, and you know where to contact me. good luck on this project. i think this is the most ambitious custom i've seen from you!! kick @$$.
btw: you're building on the wrong tower, if that matters.