I like the idea! One thing I’d suggest is making it more clear when the game moves to the next level. I noticed that in the second level (or what I assume is the second), zombies take more bullets to kill, but visually they look the same. It would be great if their appearance changed slightly — maybe different colors or textures — to help differentiate the stronger ones. Maybe show a short screen title for a few seconds like: “NEXT LEVEL!!! Zombies are now tougher!” — or something like that.
Thank you for the feedback! In fact, there are no discrete levels (or at least you would have to consider them very plentiful), since the spawn rate, health and damage of zombies increases every 10 seconds or so by a small random multiplier. I do like the idea of having a brief popup notification for this, though, so I will consider it! Some indicator for the changes on the enemies will also be added.