While I agree with the the criticism so far, I think we tend to focus on negative aspects too much, which makes the feedback unbalanced.
For one, I think the base/groundwork is outstanding and much better than many bases we see in 1/35 here. One minor criticism I would have here is that the water doesn't look like water enough, it should probably look more translucent. Another small point is that the debris of the bridge could use some more weathering to tell the destruction story better, e.g. traces of burns and rust and fragments scattered around. It doesn't look like a "fresh war wound" here.
The vehicles, in particular the Mörser Karl, are nicely painted and have a realistic look. I noticed the mesh used on the ammo carrier, which improves realism a lot. The issue with the colour scheme has already been pointed out. Regarding the vehicles in grey, to my taste the grey is too bright. The trailer for the Kettenkrad I think is a piece of scratch build. However, it doesn't look like a physically sound design, able to carry the heavy barrels without collapsing.
One word on the figures: They are not poorly executed. However, you seem to have struggled with positioning them based on the pose they have. This results in poses that seem rather random and do not help tell a story. The remedy? I strongly encourage to modify the poses. It is super easy, especially at that scale and with the hard plastic figures used here. you can cut of arms and legs of different figures (ideally from the same manufacturer) and reposition them. any gaps can be filled with putty/ green stuff in seconds. This gives you much more possibilities to tell great stories, and you avoid the pointing arm syndrome.
The overall composition is appealing to the eye as due the different heights of the base, terrain details and the massive vehicle on top. Also, the range of colours makes it interesting and realistic. The issue is that the base seem rather unfit for the story you want to tell, which evolves around the Mörser Karl. Obviously, the heavy artillery was a crucial asset in the German assault on Sevastopol. They wouldn't be stumbling around in open terrain scouting for a way forward. Also, logistics were a nightmare, so movement was carefully planned. For longer streches, the whole thing was dismantled. When at the Sevastopol front, they fired from prepared positions and the rest of the time stayed in dugouts to avoid counter battery fire. Their limited range (4 - 7 km I think) meas they were or could get exposed to counter fire. Also, the crew was sixteen men for the Mörser Karl alone.
Therefore, the setup of the diorama unfortunately is not believable. This applies to the overall situation (what is the Mörser Karl doing there?) as well as to the smaller details. It is unclear whether the Mörser is supposed to be on the move (into a deadend, obviously?) or about to fire (doesn't look ready?). The ammo carrier in the water has already been pointed out, but why is the crew operating the crane? The jeep could have been blown their by bombardment, but then it would be heavily damaged. The details don't add to convey a story, they just remain details. I guess if oe wats to tell a story, it is important to think more in detail what is realistic, and less focus on what models, figures and equipment one wants to put on a base.