Zombie Assault falls somewhere between Call of Duty Black Ops' two zombie modes. It combines the top down arcade feel of Dead Ops Arcade with the barrier defense mechanics of Nazi Zombies. For those of you who haven't played Black Ops zombies, here's a translation: it's awesome.
Zombies swarm from all areas of the map. In order to get to you, they have to break through barricades around the edge of the level. Ideally you'll pick them off before they break through, but once they start penetrating your defenses it's time to switch to survival panic mode. You'll have to scamper around the map rebuilding barricades while simultaneously mowing down stragglers. Ahh...another day at the zombie hunting office.