Hi Martin,
Really nice modelling work.
I'd recommend posting this image at forum places like
www.elysiun.com. They are blender oriented and you will get excellent improvement tips from blenderists, who are exceptionally helpfull. I'd just recommend for myself working on texturing. Tree trunks would accept some bump mapping as well as pavement tiles. (It looks very *flat* as it is now) Pillar texture looks not so good. When I saw it for the first time, it looked to me as if you've chosen incorrect mapping type in material window.
One excellent texture resource is here, where you can find almost anything:
http://perso.club-internet.fr/lemog/You should IMHO also try to incorporate Ambient Occlusion (if it's not already there) which would give much better look to the scene in general. Anyways I am not graphic artist, so feel free to ignore my comments alltogether.
Ah and the light seems a bit inconsistent. I was trying to figure out where the SUN is and didn't came to successful conclusion.
Just my 2 cents as an amateur blenderist.
EDIT: Why did you say, bump mapping is not advanced in blender? You can use up to 8 texture channels, so you can have even 7 different bump maps if you are so inclined.