You've got the beginnings of a fun action game here. Good music selection, too.
I also rather enjoyed the boss fight, which contained a variety of hazards and stages of difficulty.
A lot of polish is needed to bring this to the next level. Here are a few suggestions:
1. With pixel art, all the visual assets should have the same resolution. For instance, the tiles used in the background should have the same resolution as the tiles used for the sprites. It looks like you are using tiles with a 32x32 resolution, but scaled down to 1/9 the size of the player character's sprite. This is a bit jarring to the eye.
2. The environment should have more variety, including purely decorative elements.
3. Include a way of regaining health.
4. The Load button does not currently work.
5. I did not figure out a good use for the Dash command.
6. If making a game in the Metroidvania genre, consider the other abilities that you'll want to add, and how they will help to traverse the levels you create.