This is legit one of my favorite games ever, but I have also a few suggestions on how to improve it.
1: Make more unique levels with unusual mechanics like the elevator thing. If you want to split the game into some sort of chapters, just add one level like that to every one of them, and this will be enough (at least for me). It would be cool to have those levels at the end of the chapter (kinda like bossfights), because they will be surely the most spicy ones, and probably the hardest. Elevator thing was definitely the hardest stage of the game.
2: Fix the elevator level. I have experienced some weird weird bugs when you fall off the platform, instead of just dying you get trapped in a really weird spot. You can easily get out, but it wuld be way mor realistic if you would just get squashed and crushed by the elevator. Also when you find a way to drop a gem out of the screen, the game just triggers the next wave of enemies, and I don't think that's supposed to happen.
3: In a few stages enemies look almost the same dead and alive, and this is kinda confusing. I'm pretty sure this is way more problematic for colorblind people, so maybe you could add some sort of colorblind mode.
4: When you get hit by a pistol or the machine gun the game just freezes for a couple of frames. I'm not sure if this is the intended behaviour, but it doesn't feel so smooth as the rest of the game, and just looks kinda bad, especially since there is not such effect when you get hit by a sniper.
5: Never give up, and finish this masterpiece even if that would take years of work. It's definitely worth it.
