Throw some texture onto that slice! Buried cave network, basements, skeletons... anything that isn't just the grey void you got right now! I also reccomend working on the road gravel, right now it just looks like noisy grey flats. Add some texture and variance to the terrain!
Also, you need some more contrast on the edge bricks, I didn't even notice them at first because the shades are so close! You can also incorporate that brick to the edge of the river there!