Loved this game! It may not be the best "fit" for the theme, but it's definitely fun! The idea that every time you shot you cover a little slice fo screen, but you have to shot in order to survive, it's simple yet effective. And over that, you accomplished to implement an upgrade system, a score, a health bar, and two types of enemies! WOW :D
The only doubt I have is on the theme: why cowboys and knights? (Or maybe those are not cowboys, but... that was my first impression ^^). However, the "dark" part of the theme is there and I could really feel that :)