Thanks for all the help so far to everyone, I really appreciate it that all of you are taking so much time for writing answers

Stava was right, but I don't got his points quite from the beginning. The current version of the art should make it clear anyways

@Night:
Yeah the lightsource is always a problem. for sprites it's better to have the castshadow directly beneath them, otherwise it will cause problems with walls. The light will never work 100% right within tile-based games - I think it's more important that the forms are clear and well described.
In your example char 1 is correct and has a nice contrast, while sprite 2 describes some forms much better. I'd go with a mixed approach and rather take some artistical issues with lighting than issues which a correct shadow would cause with game approaches.
@Milokey:
Thanks for your edit, but I don't work currently on the big chars. If I have to change them at a later point I might come back to it.
-worked on the car, change dminor detail stuff as suggested
-worked a bit more on the ground tiles, changed some colors and added a tad more color variations. Nonetheless there are quite some tiles missing.
-I designed some more props
a deco tree (castshadow and some more bottom detailling is missing, also unsure if I should add branches in the foliage)
a wastebin (castshadow seems to be to large right now compared to other stuff)
a hydrant (I am quite happy with the design, I wonder if the color looks nice)
cast shadows for tree and lantern are currently missing, because I lack a good ideas how to solve them. For the lanterns I intend to add a light layer later on.

