Notsonoble, I don't know if you're wanting suggestions at this stage, but here are a few things you might want to consider:
-- you might want to increase the scale of the texture. The tiled effect is particularly obvious in the thumbnail view.
--the buildings all sit a bit oddly on their blocks. I think they'd look better if rotated to face the street, or at least rotated to different angles from the buildings nearby. Photoshop has a brush setting called angle jitter. I don't know if GIMP also has that feature, but if it does, you might want to use it.
--it is hard to tell from the image that there is a slope or mountain. I'm guessing that the way the green becomes more pale at the top is meant to indicate height? Unfortunately your city layout is inconsistent with a mountain slope. Usually, streets would wind back and forth along the slope, to take advantage of the less steep gradient. Buildings on a steep slope are also likely to be smaller in their plan. That walled compound you have about 3/4 the way up the image would be fine on flat ground, but extremely impractical on a steep hill or mountain.
Hope that doesn't sound too harsh. Good luck with it.