I like the variety of boss battles, weapons, and the area when you explore being different each time. The music is really good in this!
One way to improve could be that an ability to run holding down a key like SHIFT, even at the very beginning of the game without collecting speed boosts, might feel better. Even if it's perhaps limited by stamina (even a lot if you want in the beginning) and perhaps the stamina duration could be increased by some items, etc. To me it felt like being able to run right away might help.
Other than that though, really nice job on this unique game, it's actually really good and kudos to getting so much done in such a short amount of time!