Inspired by Zhaph's wall, this one is made with 1x3 bricks.  It has a radius of 12 studs (24 studs diameter).

![enter image description here][1]

It is connected to the base by the last two studs on each end of the arc. The middle of the wall rests on tiles.  To complete the tower you can repeat this module 4 times and use 1x2 bricks to connect them (replacing the 1x1 end bricks).

This would not make a perfectly round wall, but it's pretty close.


  [1]: https://i.sstatic.net/3Ayyg.jpg