I'm glad you finally made it to the backgrounds this time, cos this really looks promising 
As some others, I only have a slight problem with the sand. It looks a bit flat. At the back, you could change the shade from the hatch, making it more curvy where it hits the sand. Also, if you shade it on one side, shade the whole side (the sand on the lower right could need some more both at top and bottom). The sand leaning against the middle pillar should be darker. I sense you've been adding a little noise to the sand, maybe you could increase it just a little bit. Eventually, have som darker or brighter grains added here and there. In general - experiment with the structure of it (you could always look at some googled pictures for reference). Then again, it's pretty damn good as it is.

As some others, I only have a slight problem with the sand. It looks a bit flat. At the back, you could change the shade from the hatch, making it more curvy where it hits the sand. Also, if you shade it on one side, shade the whole side (the sand on the lower right could need some more both at top and bottom). The sand leaning against the middle pillar should be darker. I sense you've been adding a little noise to the sand, maybe you could increase it just a little bit. Eventually, have som darker or brighter grains added here and there. In general - experiment with the structure of it (you could always look at some googled pictures for reference). Then again, it's pretty damn good as it is.