Hi... Recursive Mosaic, Motion Blur, Emboss demo.

[maxo]

Tool: GIMP 2.6.4

Instructions
1) Use the freehand selection tool to draw a cityblob.
2) Use the Selection, "Border" option to turn the selection edge into a wall.
3) Fill the wall with a pattern or color.

4) Now... use the modified color mosaic plugin ( text included below- I modified it to support tile divider thickness).
5) Create LARGE tiles, these will dominate your city-- and the dividers form your major walls.
6a) Fill the large tiles with a reasonably grey brown building color.
6b) Fill the dividers with a nice street color.
6c) Select each of the resulting large tiles and use the mosaic tool to fill it with smaller tiles.
7) Choose the flood fill bucket tool. Select a reasonable roof color and set it to 30% opacity.
Randomly dot some of the small house tiles with 0,1,2,3 roof fills to make a natural variation.
9) emboss everything. emboss is about 58-78, depth is about 10-20 but you have to noodle with it.
10) Now- with the "similar color tool" select the roofs.
11) Apply a motion blur filter-- very light- should create the illusion of a sloped roof.
12) Mildly blur it.

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13) I created a layer of "clouds" and merged it.
14) I created a lower grassland with roads and merged it.

Should work for any size image.

The main idea is recursively going from large mosaic tiles to house sized mosaic tiles.

Oh yea-- I also used the perspective tool to tilt everything when it was done. Also, note the foreground road is brighter while the background roads are grayer. I tossed in a couple park areas and a lake while filling large tiles.
The brown areas are reserved to put a castle in. The parks need to be darker.

Entire image took me about 50 minutes to finish. I've been using GIMP for about a month so any one should be able to get similar effects. Give it a whirl.