OK, let's start with the cast shadow. The light source you have is coming from behind the wall judging from the directions of the cast shadows. In order to make the cast shadow lighter than the form shadow (which is unusual), there has to be more light bouncing into it. Your secondary major light source is the sky, which is having a pretty minimal effect on the image right now - it is a fairly strong, but very diffuse cool light source. On a bright day like this, the cool sky light would be hitting the wall, the shadow, and that plinth the guy in the foreground is standing on, so you may want to strengthen that. Due to this, I think the wall would probably be lighter than the cast shadow in reality, but if you feel keeping the wall darker helps the composition more, perhaps it is best to keep it that way.
Also, before I forget, remember the wall shadow would become more diffuse as it drops further from the wall due to the light scattering around the edge.
There is a bit of a lack of variety with your textures as well. That is kind of the thing that stood out to me at first - there is a bit of texture to the ground/stone, but the way the light reacts with the materials is fairly uniform in the image. It is a bit tricky with such a drastic light source, but I think you would have more tight, specular highlights on the metals - and stronger fill lights from the sky. The stone objects, especially in the foreground where the guy is shooting, could have more in the way of imperfections as well - small notches or gouges to break the straight hard edges a bit, little colour variables and the like.
The final note I'm going to make on light (sorry for focusing on it so much! the rest of the picture is pretty solid though), is that your tertiary light sources - fire, energy beams, etc, aren't really emitting much. They are small sources compared to the scale of the picture, so they would indeed have a rapid fall off, but they would definitely effect the appearance of local materials. So for instance, that fire on the wall would give the local bit of wall a warm glow from the light it is emitting. The guy in the foreground is shooting some kind of energy missile, but the barrel of his gun has no light from that, despite how much the beam is glowing. All of these things would create small amounts of colour lighting, and it may help to add that in order to ground those elements into the image.
Compositionally this is working pretty well, it has a really clear read. I like how the monster and the guy on the plinth are pointing in an the giant robot, as well as the repeated smaller pointing cues from the army, and the way the gates frame it as well - it creates a very clear focus for the viewer. Your values are pretty clear too; they divide the image up into zones quite well, and don't confuse the eye. If you wanted to improve upon the composition, it might help to delineate that front line of monsters and turn it into a more undulating form to add visual interest. The snout of the really big creature might benefit from a few rim lights to pop it out of that wall shadow as well.
Finally, a small thing with construction and perspective. The angle of the far corner of that foreground plinth seems to be a bit too obtuse, and is warping the perspective a bit. Otherwise the construction seems to be fairly solid, and the horizon is pretty clear to me, so well done on that.
Overall this is actually a pretty strong image, and checking it against some of your recent stuff on dA, really shows you are pushing your skills up a very solid notch, so congratulations on that. It looks like you really put in a good effort to push yourself here, so well done!
Thank you so much for so constructive critique! It helped me.
But I don't quite agree with your statement that small lightsources in the picture should
emit much more light because it's sunny day and light from the sun is thousand times more powerfull than lasers, fire, beams etc. So i think we won't see too much glow because light of the sun just pushes away (maybe it's not the best analogy) other lightsources. But it's just my opinion.
It helped me.