Lots of bugs. When I got the green gun, bullet sprits would freeze on impact, so before I reach a half of zombies killed , the screen was full of those bullet sprits. Then when I used pause button, the pause menu appeared but the game didn't stop.
For the game play what is the point of permitting to shoot 360° while this is a one dimension game ? I did not notice that there was a way to do headshots. If there is, a little graphic *headshot* would be nice.
Then the difficulty raises on the first levels with new enemies, but then only raises thanks to the number of zombies. Each stage becomes longer, not really more difficult, and as a result, more and more boring. There should be new enemies at every stage until you reach the best gun (and only then, an endless mode with no new enemies).
As the game play and the game design are very simple, it can't entertain more than 5 min. Make things so people finish it in less than 5 min with a finish screen, a total score.
As such, this game is not interesting.