It's an OK concept, however I do have some thoughts:
First, some of the buttons on the menu do not work, such as the tutorial button. Also, during gameplay, I do not noticibly see the "cooldown" on my gun, if I hold down on the mouse, it'll stop shooting, I'm guessing it's "overheated" but I don't see a meter for it (or a working one for it.)
I also could see where my gun was shooting but I could not see the enemy's rounds being traced, so I heard the sound of them shooting but couldn't see it.
Perhaps next time add health packs or regen health. Also weapon drops would be nice.
I also noitced that once the main music stops, that's it. I did refresh my screen and got a different song each time but it seems that the music just ends with that one initial song. I suggest extending the songs throughout the game.
When I died, I had no lives but that initial one. After I died, the enemies were still spawning in the game and there wasn't a "reset" button.
Hope this helps you in your next future endeavors.