I like the style of the game!
You can probably make it a bit more challenging easily by introducing a reload mechanic, because at the moment I think there's nothing stopping the player from just clicking the whole time to kill everything that shows up on the screen. Even better would be if you make the weapon only be effective when the enemy comes close enough so that the player has to time their shots and gets hit if they are too early/late. Adding in a bit of movement for the player would add even more possibilities.
Very cool sound effects, gave the game a real sense of atmosphere. I also liked the small touches like the stars shining and dimming and the different layers of ground with bones and stuff. I like the zombies animation. Good work!