Well, you have definetly gotten off to a good start. I would say that there is room for improvement - one thing in particular that I see being realism. What can help too when you are coming up with a design is looking at reference material for ideas, as well as trying to imagine in your head before you start building what you want the final outcome to be.

There are are many different build styles you can use, and even can come up with. Keep working on it :)

Cool tower... if I manage to get inside I could probably kill you though. (And with TNT I could easily kill you!)