Let me start off by saying that it isn't very clear that the green boxes are the zombies. For me the main problem with the game is the shooting. I think you register a hit if you click on a zombie and the actually bullet is just an effect. However it is extremely hard to click multiple times on the same zombie when you're running across the level trying to avoid them.
I didn't really see anyway that the objects interact with each other. Perhaps you can make the bullets actual GameObjects, so that they can bounce of the walls. You can also make the barrels explode and zombies bounce from wall to wall.
Sebastian Lague has a good tutorial on how to make a shooter game, if you want to improve the shooting. Whenever I make a game in a new genre, I start with a tutorial to understand how the basics work and then I build on top of that.