It was fun to play. The mechanic that you have more damage when low on hp (and also the walls) is interesting. Except for the part when I chose to get 85hp and was full hp so I had to get damaged almost to 0 again just to get further. So basically a good player that doesn't get hit can't progress except he runs into spikes.
The music is simple but while playing it was not annoying. It suits.
The enemies at the start (and their bullets) are way too fast. I think the first enemies should be almost dummies, so that the player can get used to the controls and so on. After playing it through I was able to not get hit by them but for a new player they might be too hard.
There were some issues like, when choosing an upgrade, new enemies came from somewhere and attacked you, so you have to fight them while having 3 big upgrade boxes infront of you.
Also I had a really hard time to click on the upgrades. I had to move the character and the mouse and search for the hitboxes, sometimes it helped but most of the time I only could click on the middle upgrade.
Overall it is a good game and I would love to play it again when the bugs are fixed and the game has more content. Good luck :)