101stAirborne,
A shadow box style diorama world really work well for your idea. Force the perspective using different scales of figures positioned front to back. There are few photos of sewer fighting from WWII, being dark and vile places the camera crews realized the pictures would not have turned out anyways. Include brown water, rats and desperate looking soldiers.
The Soviets used the sewers in the battle of Stalingrad to out flank the Germans attacking from behind their lines, possibly the German veterans of that battle realized the strategic value and exploited the sewer tunnels for any edge in the final days during the fall of Berlin.
Include mixed arms, uniforms and ages of the soldiers. A small squad of 6 running towards the viewer would be really effective to highlight your idea.
Good luck, as always looking forward to some cool WIP's!