Looking forward to seeing how this turns out. It's looking pretty good so far.
I think the wadi looks great, very convincing! Wouldn't mind knowing how you achieved that look if you are willing to share.
What *is* the scale of the city? If it's a very big city then your wall might be a bit thick and the sand and rock textures you're using might look better scaled down a bit.
How are you doing the rock/sand? I often use a tileable texture as a pattern overlay on a completely filled layer. Then I mask the whole thing out (filling black into mask layer) and then paint back in the parts I want to show in that texture (by painting with white into the mask, usually using a "natural media" brush of some sort). You can then change the scaling of the texture, add bevel/emboss, drop shadow, etc. as desired and you've got a sort of "non-destructive" method of playing around. Please forgive me if I'm telling you something you already know.
Also, for my ground, I usually use several such layers: usually a cracked mud base, completely filled; sand or dirt (or both) painted in on top of that, mostly covering the cracked mud, but letting it show through in places; then I'll usually put some groundcover (which might not be applicable here) like grass, rough but low vegetation, and so forth; I'll top all of that off with some higher (by moving the shadow and amping the bevel/emboss) plants, bushes, and trees (again, maybe not applicable, though you might expect to see a bit growing in the low or sheltered areas even in a desert.)
On the wall: it might be a bit thick, as mentioned above. It could also do with a texture of some sort, even if its sandstone. How's it made? Should we see some indications of blocks? Do you want any color variations? Your method of construction appears fine to me, but the coloration leaves it a bit cartoony. If that's not the effect you're going for then some texture/variation might help it. What about towers, guard posts, gates? Some indication of those would break it up. Finally, you should think about your shadows. Instead of using a simple drop shadow I'd copy the wall base, move it into position and color it black and then paint in any gaps between that and the wall. When happy with it I'd use an appropriate level of gaussian blur and the set it on "Multiply" with a mid-range opacity (I tend to use 50% or 66% but, if you want to imply harsh sunlight then a higher value might look okay too.)
Okay. That's it for me! Hopefully you find some of it useful. I just completed a desert-ish city (posted here) and would be happy to share any of the techniques I used on that with you if you're interested.