It looks like my feedback is pretty similar to what others have already talked about. The game idea is solid, but it's just too easy to play. The game would benefit a lot from some kind of increasing difficulty system. Something like causing more enemies to spawn as time goes on, starting to spawn strong enemies after a certain amount of time (these stronger enemies should look visually different), or causing the enemies to move and spawn faster as time goes on.
Something else that would help make the game more satisfying is adding feedback when you hit and defeat enemies. Things like sounds and animations are a great way to do this.
Good work though!