The design is simple and effective: while maintaining minimal art, I enjoyed the presence of key animations (enemy death, grabbing items), since they make the game modern despite keeping a retro feel.
Also good choice in making a boss appear sometimes, as it adds a little variety and poses a challenge.
If I may suggest some feedback, I would add multiple enemy types (slower, faster ones) and give the boss a little feedback on the fact it has been hit (such as small particles), but overall it was a great job, the game worked and I haven't encountered any bugs (got to a high score of 178)
Thanks for the submission, all the best! :)