This game has a lot of potential. But for whatever reason you decided to only put a demo here when the site provides dozens of other defense games that offer 10x the amount of content.
If you want to make this game into something good, you should add some more content, and upload the full version as well. Base upgrades would be nice and blood stains on the ground from the dead zombies. You also need to make the enemies get consistently stronger. If I can head shot the supposed stronger enemies with the base level rifle, then the game just got to easy.
Overall the game us fun, but needs a lot more. More boss battle and everything. The art is good, just more content is needed.