It's a little bit limited in terms of "gameplay" - one way to keep your player engaged is to think about decisions. Games are pretty much always a series of decisions - where do I go, what do I shoot first, etc. Your job as a game designer is to make those decisions interesting! I didn't feel like you really achieved that here. One way to do that in a game like this would be to have enemies with higher health, or ones that drop more quickly :)
I feel like the main mechanic (shooting) could do with a little more juice. Bigger shots, faster fire rate - you can compensate for the player's increased strength with more enemies 😁 you also need some visual feedback - sprite flashes and/or particles when the enemy gets hit, for example.