Fun game! Had a good time with it! Reminded me somewhat of Crystal Quest I used to play on my first Mac!
A thing I didn't like as much is that the spawn distance of the skull enemy feels short, especially later in the level when things speed up and they pop up every few seconds. When you're focussed on the rapidly spawning skulls, you can't pay much attention anymore to where you are firing your blades to.
Furthermore, when you're enthusiastically firing all your blades, and you can't fire any more because you haven't gotten any upgrades yet, you feel like a sitting duck when the skull suddenly spawns. The strategy then would be to always keep one in reserve, but that doesn't feel as satisfying unfortunately.