There are several problems:
1. The zombies are too fast and it takes a lot of shoots to kill them. I didn't last a minute at the easiest level.
2. I usually do not criticize the graphics, but this time I need to make an exception. That's because when the zombies killed me, for a moment I thought there was a bug that did not allow you to return to the main screen (counting the fact that the character keeps firing even when he's dead). The action buttons (restart and menu I guess), are barely visible and also do not seem like action buttons, it seems that they are part of the scenario.
3. A game without music is terribly boring (at least for me), unless it's an Atari 2600 game.
I hope this review helps you to improve.
Sorry for the bad English.