Thanks for all the comments.
Ulco, yes probably the terrain beyond the camp it's too barren and creates an abrupt unbalance: initially I wanted to create a more desertic and arid area, but then got drifted away putting too much dry grasses on the front.
About not using TG to render grasses, the answer is that I'm trying to learn new techniques like speed and matte painting, compositing in order to produce images at a higher pace. I know there is a long diatribe all over 3D sites where people discuss the "purity" concept of 3D art, i.e. untouched renders vs. postworked and composited ones.
Personally, as I'm getting more and more little work as landscape artist, I'm in need to produce images quickly and so merging base renders with real-life photographs or 3D elements coming from other renderers is a step I need to take.