I enjoyed this little demo. Thank you for this!
Here's couple things I found odd or something to improve.
- Odd placement of fullscreen button, I think this should be in the options menu. Now you can't change that setting without exiting game unless you know that alt+enter works.
- Options menu should have some indication how to exit it. I found out that ESC works but since some games doesn't save setting when exiting with ESC I had to go back to see if changes are actually saved.
- It wasn't obvious that you'll need to use mouse to aim. I tried to aim with keyboard w/s/a/d. Maybe you should add option to play with just the keyboard.
- This one is pretty niche problem but when playing with 32:9 aspect ratio, there are huge black borders. I think you should add support for other aspect ratios as well. In this game having wider aspect ratio wouldn't even give player any unfair advantage since there aren't any level designs that rely on hiding stuff by scrolling those out of the view.