I managed to put together the following set-up for the beginnings of a slightly narrower tower:
This gives me a radius of about 7 studs if I go as tight as I can, giving a diameter of about 14/15 studs. I would have used [part:3062b] but I'm not that organised (yet), so went with [part:4589:378] instead.
I'll have to see what I can do with 1x3's and cylinders to get closer to 20-30 studs.