Well what you have is the top of the tower...what's inside of it like the rest of the castle? I'm guessing a spiral stairway so just make some outer walls then put stairs in the middle part. If it's a non-functioning tower, and is indeed solid, then what you have seems fine to me but you need battlements (those square thingies that are spaced out so that archers can hide behind them and shoot down dudes trying to scale the walls).