Well, I am hoping that this grunt thing is not totally subterrainean. WWII did have underground bunkers, but not a whole hell of alot.
I would prefer that the infantry bunker complex is vehicle proof, but still mostly above ground. Ring it 360 degrees in anti-tank minefields so vehicles cannot approach the base, but infantry can walk right thru it, and then use trenches with gun bunkers and such with some underground tunnels. Put it on top of a small hill to further protect it from vehicle guns.
All of which is realistic too.
Still, most FPS games are run as indoor type of things. It won't kill me if it is all undeground in a Maginot Line syle base thing.
Hans.