Because Zombies is a tower defence game that twists the genre a little. You're defending yourself from the undead, placing turrets and traps to keep them at bay.
Everything is controlled with taps, and the zombies you kill drop bags of cash that you can spend on building more defences.
There are machine guns, spiked floors, barricades, attack dogs, and a variety of other undead manglers at your disposal.
It's a reasonably familiar game, but if you're looking for an undead botherer with a bit of a twist you'll probably find something here to like.
You might also like... Plants vs Zombies, Anomaly, Kingdom Rush