That's right! The core of such a tower is a gear. For European builders, it's an Expert Builder gear from the 70s, e.g. http://www.bricklink.com/catalogItem.asp?P=g21
which comes in three sizes, combined with Znap pieces from the 90s, for example this: http://www.bricklink.com/catalogItem.asp?P=Zbb015
. You can easily fit the end of a Znap piece onto the tooth of a gear. The items needed are available at a moderate price in many Bricklink shops. One advatage of this technique is that you only need two gears to construct a tower.
For fleshing out the tower, you'll need to insert half pins http://www.bricklink.com/catalogItem.asp?P=4274
into the Znap holes. Then there comes as many strips of x1 plates and/or bricks as there are teeth on the gear. For a 21 tooth gear, the height of each strip will be 2 bricks. When you've applied the strips onto each Znap beams, simply cover the strips with 1x2 tiles in an alternating pattern like this: http://www.brickshelf.com/cgi-bin/gallery.cgi?i=4824121
Of course you can use 2x2 tiles and 2x2 round tiles or any other 1x2 modified tiles or modified plates for a more exquisite pattern, I leave experimenting to you
. The diameter of a tower on a 21 tooth gear-Znap core will be almost exactly 14 studs. For a slightly smaller diameter use thinner strips and 1x1 and 1x2 tiles like here: http://www.brickshelf.com/cgi-bin/gallery.cgi?i=4553929
For a slightly larger diameter, use thicker strips and 1x3 tiles. You can vary thickness along a strip pretty easily. This also comes in handy when adding a window or an arrow slit. Remember that a window will inevitably break the tile pattern which you'll have to arrange somehow. For the roof, add hinges e.g. http://www.bricklink.com/catalogItem.asp?P=30365
to the strips along with the other ends bent in an angle and cover them up with wedge plates http://www.bricklink.com/catalogItem.asp?P=41770
or similar ones for steeper roofs. The funny thing is that you won't inevitably need pairs of "left" and "right" wings, a stock of wedges of one orientation will do. Arranged in a circle, they will cover almost everything.
...So, cunning trick, huh?
The other technique is not much less cool and is aimed mainly at American builders who have a slightly wider selection of old gears from the Samsonite range, like this one: http://www.bricklink.com/catalogItem.asp?P=742
Unfortunately, these gears are more difficult to come by, and they're almost exclusively found in American Bricklink stores. They also tend to be more expensive than Expert builder gears.
Anyway, towers built with Samsonite gears are VERY stable.
About a year ago, I constructed a lunette http://www.brickshelf.com/cgi-bin/gallery.cgi?i=4086655
on my clock tower applying a Technic hub http://www.bricklink.com/catalogItem.asp?P=424
on each tooth with alternating orientation. However, this is not the best solution for bigger towers as it is a bit weak and shaky, let alone expensive.
For sturdy spires, you will need inexpensive pin joiners http://www.bricklink.com/catalogItem.asp?P=62462
or modified plates http://www.bricklink.com/catalogItem.asp?P=2817
I prefer this latter one because they will stabilise your tower even further by connecting them with plates. However, I'll focus on the simpler build here with the pin joiners and let you experiment with the latter.
So, the first problem arises: you can NOT place pin joiners on adjacent teeth. That can only be done with expensive hubs. This issue kept my thoughts away from the topic for a year. Then two weeks ago it just struck me and I finally got it. For a tower, you'll need at least four gears: two for the bottom and two for the top.
Let's take the bottom, the top will be done in a similar fashion. Get a gear and attach a pin joiner on every SECOND tooth. Then stack one brick and a plate on the top studs of the gear and connect it with the other gear. Take care that all the teeth on the two gears are parallel to each other. Now you should add pin joiners to the second gear, putting one on every FIRST tooth. This way you'll have a bottom section. Now finish the top section in the same manner.
From this point on, there are two ways, but as a matter of fact both ways employ the same strips I described in the first part of this article. The less expensive version includes a studs-down application of the strips with 1x2 plates on top. This way, you save the half pins and achieve a Renaissance sgraffito look: http://en.wikipedia.org/wiki/File:Zamek ... czynie.jpg
The more expensive way is mounting a studs-up version on half pins, like with expert builder gears/Znap beams.
Samsonite 14 tooth gear
height of strips: 2 plates +1 tile
tower diameter: 10 studs.
Samsonite 21 tooth gear
height of strips: 2 bricks +1 tile
tower diameter: 15 studs.
Samsonite 35 tooth gear
height of strips: 3 bricks 2 plates +1 tile
tower diameter: 23 studs.
Samsonite 42 tooth gear
height of strips: 4 bricks 2 plates +1 tile
tower diameter: 27 studs.
I hope this article was informative to you.
I'm wishing you a happy experimenting with round towers!