Alright, overall it did seem to be a pretty good game. I was playing it without a mouse but that obviously won't effect your rating, just take note for this.
I really liked the style if art use in the game and the audio seemed to be quite well suited. The game play in most cases seemed smooth as well.
Being said I found a fairly large issue that I had with the game. You could walk up and down. Don't get me wrong I love side scrollers that support this but there is generally an auto aim up and down the lane based of how high/low you're aiming the weapon. It can cause irritation at times but in this case it would've fixed more then it broke. I've just found that sometimes there were no zombies in the line in front of me but I was standing on a slightly lower platform that bent up, unfortunately I had to travel towards zombies that were coming at me an had no choice but to either wait or engage them at a closer range. Although this wasn't game breaking considering they weren't a huge threat unless massed spawning seemingly.
Aside from the down line aiming the focus/aiming in general worked great. Without a mouse I managed to tolerate it fine which I'm honestly quite impressed with. I absolutely hate track pads but I was able to soldier through this game with one no problems. You nailed the controls themselves.
Another thing I liked that you did with the game was set drops in barrels and seemingly none of zombies (I haven't seen any.) When your trapped in the survival area's it forces you to feel obligated to avoid all damage, who knows how long your going to be stuck or how long it will take you to get the door open, it legitimately makes you work towards progressing through these areas rather than knowing if you last long enough a health will drop off a zombie and you'll be able to walk on as if you aren't getting hit.
One thing I would recommend would be decreasing zombie health and maybe increasing their damage. Really you did nothing wrong here but I'm fairly certain the mass likes dropping the enemies a little bit faster, and being force to or else they take massive amounts of damage. This won't lower stars because it's strictly preference, just a thought.
You have a good game here and it definitely looks like you put work into it. Keep it up, it is a damn good game. A little bit of patching could improve it but what can't be improved?