ideas for the future:
Hidden hallway unlockable by a code, goes to another small bunker/outpost around the level that holds a mannable turret (thanks powermad!)
3d skybox
ladder going up to the top of the tower with some sniping supplies
Bosses:
strider
group of fast zombies (jumping around the base would be cool

antlion guard and antlion swarm (kill antlion guard to stop the swarms attack)
Events:
Mortar raid (random strikes all across the map like artillery fire)
Zombie round (mass zombies!)