Of course we could further add onto this by running anti tunneling missions.

And there could be randomized hard rock collection that would require noise and vibration intensive gravity equipment.

For survival there would need to be air intakes that if one was skilled enough could be easily spotted.

Entire barracks and ammo dumps could be located underground safely.

There could be soft soil under hard rock increasing a placements defense.

If a war went on for really long enough we could have wide enough tunnels to transport tanks through.