HI there! Your game is very simple and runs well! However, I think it can improve significantly! Add some more animations or some variation is zombie movement or guns! Your game actually reminds me a lot of boxhead/boxhead2play (if you've never played it before check it out!) its a flash game from a long time ago and it does what your game does but just all around is more engaging to the player. Sorry if this seemed harsh but I think you can improve this significantly and would love to see you do so! I also think having the bullets go through zombies and the ability to hit multiple zombies at once make it too easy!