SAS: Zombie Assault 3 is another free micro-transaction game by NinjaKiwi for Android and iOS. It's a top-down L4D-esque shooter with some tower defense aspects. I'll say right up front that this game would vastly improve with controller support, but on a tablet, it's still very playable with the large touch screen. You start off with just a pistol (unlimited ammo) and fight off the zombie horde in an old farm house. Strategic points of ingress are barricaded off, but the zombies will tear them down. You must keep up with repairing them as the game progresses. Completing missions and killing zombies earns you money and experience for higher levels, new maps, and new weapons. There's plenty of zombie varieties, too... from the shambling Swarm Zombies to fast-attack zombies, and their versions of Boomers and Tanks. There are online multiplayer modes including 4-player co-op. Of course, there's the constant push to buy the full version, but you can play quite a bit for free. The fact you can go multiplayer in the free mode is pretty awesome.

Zombie Assault 3 is available in the Google Play and App Store markets.
The official site also has a browser version to try with Ye Olde keyboard/mouse:
SAS Zombie Assault 3, with multiplayer