Of course the more zombies you shoot, the more aggressive they get. They start off groaning and stumbling around. You move by tapping a location on the screen and then tap on your target. The closer you are, the better chance you will dispatch the zombie with a fatal shot. There's something in the plot about rescuing a beautiful princess, but I was overwhelmed with zombies before I could get to that point.
I actually liked the game. If the zombies get too close they will try to strangle you, and your death halts the game -- and your life. You start again from your previous highest level. The graphics are good, and the audio is good too, with appropriate sound effects and lots of groaning zombies.
There are some challenges in navigating the terrain. There are fences you can't get over and sometimes the vegetation makes a particular route impassible. The animation is smooth, and certainly did not tax my third-generation iPad.
Zombi HD requires iOS 6.0 or greater and an iPad.