im really likeing how this is shapeing up.
you are making good use of the colours, and detailing well.
I made a quick edit though for you:
first, i changed the rocks at the top. I felt yours were alittle too soft, with not much form. So i changed them alittle bit, and used more shadows to develop the shapes better.
then i played abit with the side wall (on the left only, i didnt touch the right side).
I again made the rocks more rock like. Your wall seems really soft and the rocks just flow together, instead of being separate entities being squished together.
I also tried to extract your grass tile. That may not be exactly the tile you used, but I tried to find it. here you were really well with detailing and making sure everything stands out properly. However, this should have been more focused on the rocks. Generally when you look at grass you dont see every individual blade, rather you see grass as a whole, and where the shadows and highlights are. So i broke up abit of the individualism of each blade. Also, I rearranged the shadows and highlight. In your version you used the shadows for the edges of the blades, and the highlights to accentuate. What i changed, was so that the shadows were more on the bottom (and middle after first set of blades) and the highlights just on the tips of the blades.
With the rocks being harsh and prominant, and the grass being more random, it creates a nice balance, instead of it all being harshly overbearing or too soft to distinguish anything.
Just an example of how I saw it might be able to improve..
