If you're unfamiliar with the mode, it places a handful of soldiers in an enclosed space against waves of progressively stronger and more plentiful zombies. As you blast the hordes and repair damaged walls and board up windows in your makeshift fortress, you're able to buy stronger weapons to fend them off. Make no mistake, you will die. Your final score only reflects how long you were able to hold off this inevitable conclusion.
On the iPhone, two of the maps, Nacht der Untoten and (for an additional $4.99) Zombie Verrückt have been brought over almost exactly from the console. (If you're a fan at all of Nacht, I'd recommend picking up Verrückt. It's maybe a little on the pricey side, but tweaks like electric barriers help the experience to feel way more fleshed out.)
The main difference from the consoles is that sending the undead jerks back to their graves is a lot easier with an incredibly generous aiming assist. While it might seem like a game breaker, it actually just allows you to focus on weapon purchasing, keeping the zombie horde thinned and keeping your barricades in place (which are really the game's strengths) rather than getting frustrated with how difficult it is to aim with your thumb in the way.
This makes for a lot of fun solo, but it's at its strongest when you're playing 2 to 4-person co-op online or over WiFi or Bluetooth. Sure, you miss talking to your squad mates, but the core of the console experience is definitely there. Yet another reminder that with enough elbow grease, it's amazing what you can squeeze on a phone.