I could definitely see this going from a good game to a great game if more upgrades + enemies were added. The music had just enough variation to not be repetitive after 20 rounds, which is nice. One thing I will say is that the game ran perfectly when I first started but around levels 18-20, even with no enemies or pickups on screen, it started stuttering. I'm not sure what could be causing that.

This is a very fun game! The theme implementation was very well executed. The upgrade system also leads to some very strategic plays, depending on how much time you have left. Overall, very fun game!