Call of Duty: Zombies was a hit on the iPhone, a simplified port of the popular “Nazi Zombies” game from Call of Duty: World at War — players fight to defend a building from increasingly difficult waves of advancing undead. It was a fairly humble iOS first-person shooter, but it had a strong brand behind it, was fun, featured halfway decent FPS touch controls, and even gave players the ability to team up online with up to three friends.