Hi,

Regarding lighting there are three things jumping to my attention:
1) The intensity of the light falls off in the background resulting in an unexpected dark background.
2) The towers don't cast any visible shadows.
3) The light color is pure white.

So what you can do about it (Best thing you save a backup, and start with a new light setup )
1) I guess you used a normal lamp or a spot (or even several of them) here. It is better to use the Sun light-type here, because it is casting directional light, each lightray being parallel to the next.
The sun is thousands of miles away so the light won't loose all it's energy in the last few meters.
Just place a lamp somewhere (Remember: it does not matter where that lamp is placed, just it's rotation is important) and rotate it as you like.
For example letting it point it a bit down and towards the towers will result in a long shadow typical for sunny afternoons.

2) Enabling the 'Ray Shadow' Option for the Sunlamp will already do the mostly trick here I think. Just don't set the Samplerate too high. Believe me. Not a good idea.
3) On a sunny day the light has orange tint, due to the atmosphere filtering the blue color. So give your newly created sunlamp a slight (underline slight! ) orange tone.

If you going to render now the scene you will get some heavy contrast because you only have one lamp and very dark and very bright areas. So we gonna need a new lamp casting indirect light casted from the atmosphere. This can be done in several ways:
- by duplicating the sun lamp, (Shift+D not Alt+D), rotating it and changing the light color to a blueish, darker tone. Also make the shadows much softer.
- same as above but use spot lamps with buffer-shadows instead of suns and duplicate them so they light your scene the same way.
- Use an HDR image as light source (The way I personally prefer).
Choose one you like from `smashmethod 's collection [link] and load it as Skytexture and map it to the sky. Now enable AmbientOcclusion (sadly you need the raytraced version) and use the 'Skytexture' option. Try both 'Add' and 'Both'.

Some tipps you might find useful:
- You gonna need a fast preview of what you're doing. So set OSA to 5
- Add a Claymaterial (Just a normal default material) and set this one to be the only material by adding it's name in the Renderlayerbuttons (F10 in the buttonpanel) and fill it in the 'Mat' field. This will give you a better control of what you're doing as you don't have to worry about breaking the material settings.

Oh and I think the grass needs some more color variation I think (and a more yellowish tone in general).
So far so good. Ask any question that you think off and I will try my best to answer them.