Mind-boggling
So this game was released shortly after another zombie defense game called The Last Stand. That game paid more attention to variety in the zombies, and lasted for a set time (30 days). What this game lacks in quality compared to The Last Stand, it makes up for in quantity. The magnitude of the violence in this game is unreal. In 2 in-game days, I killed 4000 zombies, using weapons like a Desert Eagle that holds 39 rounds, and a fully automatic shotgun that fires 30 rounds per second. In other words, there was a lot of weapon variety, but they were tailored for gameplay rather than realism. I ended up quitting long before I would've died, though I could pick it up again thanks to the save feature.
The gore level was too much for my tastes. I don't need the entire playfield literally covered in blood and shells. I'm lucky it didn't start slowing down my computer, but as it was I could barely see the live zombies at times, there were so many corpses around. Going for that much blood just doesn't appeal to me, which is why I'm giving only a 5 for violence; the violence score isn't just a measure of quantity in my view.
It had spooky ambient sounds, a natural interface, and high scores. Though in the heat of battle, I don't want to be reading numbers, so I would've used color or flashing in more places to indicate things like when you're running out of ammo or when your base is taking damage. One nice touch was that you can only select the best (pistol, revolver, machine gun) you have, so you're not scrolling through 20 different guns to find the one you want.
Here's a tip: get the Desert Eagle (4000 XP) and Overkill (1500 XP) as soon as you can. The Desert Eagle does so much damage that turns into experience with Overkill, you'll easily rack up enough experience in the next level or two to buy whatever you want.