How does your submission match the theme?
The concept here was a gun moving as if on a conveyor belt, with shooting also changing the direction the gun is moving. Later in development, I added health tanks that, when at full health, add to a hidden experience variable. collect enough health tanks, the enemies get faster and harder to kill, you can shoot faster, and your max health increases.

Third-party resources
This was a solo project, made with Game Maker Studio 1.4 on Windows PC (I'd provide a link to the license if I could find it; since I can't, here's a link to the company's site: https://www.yoyogames.com/ )

Contributors
Made with moral support (and beta testing) from a friend called Veronica Pauli.