Thanks sPlKe, you're precisely right with the posterisation effect! good catch!

What puzzles me though is this feedback I got at indiegamerforums about "The textures are very jagged". Does this mean that they are low resolution, as sPlKe also said? Is this noticeable on the top-down (ingame) view also or just when you're too close (in briefing screen)? After all, a resolution 512x512 for the building textures is not enough?