First of all. I made this tutorial in November 2008, but somehow I forgot to publish.
In April I described how to design outlines for pixelart purposes. Today I decide to show how to break “square” outlook of tiles. So we will challenge an almost inmposible shape – spiral tower. Moreover that spiral construction will be tilable and allows us to build the tower with unlimited height.
Tools:
- Inkscape (www.inkscape.org)
- GIMP (www.gimp.org)
1. Lets try to draw “gear” in Inkscape. We will use for that [polygon] tool.

2. Duplicate our gear-shaped object, and rotate it.

3. Convert it to perspective view by changing height to 70%

4. Now add colour (I think white is best choice) to each “gear” and move it like this:

5. Now all we need is just correction of distribution. We do that because we want our tower to be vertically tilable. Remember that 32×32 grid is very helpful. Notice that I mirrored whole shape, because right side of it was darker – and now it will be easier to manage correct light and shadow effects.

6. It’s done. Shape on the right is our base tile. And who said that was an “impossible shape”?

If you got any questions, just ask in comments.