I like the art direction and scenery. There is a bit of a difference in resolution between the background and the player but that might be your personal taste. Nowadays, art mediums are mixed around!
Music would have definitely given this a little bit of that juice. Getting 50 coins to reach the boss was a bit long and didn't give me incentive to try it again to reach the boss.
Having a visual queue that the boss arrives would be nice or slower bullets coming at me at first to give me a sense of how this boss attacks. It was all of a sudden where bullets started hurling at me and had me panicking lol.
Great entry!!!