OK, keeping in mind this is a scuffed version:
The boss fight is super easy. Like, far too easy. I consistently defeat it without moving at all, and it doesn't feel like much of a challenge. But the idea of fighting a boss on a crumbling dungeon floor is good.
At first I thought the absence of sound effects wasn't a big deal, until I realized the music is rather short and doesn't loop. This would be a major improvement.
I really dig the visual style, though you might want to make the boss and its bullets stand out a little more, with a black border for instance. The game is readable as such, but a little more contrast wouldn't hurt.
Anyway, good job!