Roads and city terrain are located by perlin noise. Natural terrain is left alone everywhere else. Cityscape only works with the new "valleys" mapgen at the moment, because the road noise has to match the terrain. It may be possible to accomodate other mapgens in the future.
The source is available on github.
Note that the original code is still available in the "original" branch on github.
Code: CC0 and others, textures: CC0 and others
Mod dependencies: default, stairs, doors
Download: https://github.com/duane-r/cityscape/archive/master.zip
Recipes: