Night of the Living Dead Defense is somewhere in the middle: not the greatest tower defense game you ever played, not even the greatest one with zombies in it, most likely. In fact, it's kind of a big ripoff of GRave Defense. And like most spinoffs lately labeled with the name of a classic Romero movie, it's got jack squat to do with Night of the Living Dead.
But hell, it's free instead of two bucks, just for today, and provides yet another way to spend another hour of your life shooting zombies. And you can't buy that kind of quality time, really.