very enjoyable overall. some comments: i played until the end just to see what sound the great one would make when i popped it (because all the other circles made cool sounds) and i was kind of disappointed. also the process of killing the boss is easy yet takes a long time if you max out on everything else, it would be nice to have a more interesting mechanism that maybe doesn't take as long. (e.g. roomba and rocket seem not to affect the boss? which was sort of disappointing) it would also be cool to be able to upgrade the roomba and rocket once you max out on your own turret.

pretty cool, way too easy though; all that's really being tested is reaction speed, maybe if there was more to the gameplay like alternate firing modes that are more effective to different coloured targets or something. I also agree with everything the other guy in the comments said, fix the problems, add more content and you'd have a great game

easy nice fun game

I believe you do have a solid base here. However, I am only giving you 3 stars since this game's upgrades feel way too overpowered. I think it has to do with the fact that there is no progression towards enemy spawns. You just often get the red circle crowd spawn way too often, which gives a lot of money for a short time. Because of this, you get money super quickly and it doesn't feel rewarding.
The regeneration upgrade also is too overpowered. It gives lives at a way too fast rate to the point where I am at 33 lives. Even with the one hit enemies, it still feels too easy difficulty wise once I get the piercing upgrade. Maybe enemies that require two or three more hits even with the upgrade would at least add some challenge to this game. As it stands now, there isn't much incentive to continue trying since the game is too easy.
I will give you props for the graphics though. It's definitely not a masterpiece, but I do like the old-school flash aesthetic, and I'd say you nailed it here.

Overview for this messy review: Either raise the upgrade prices, limit the fire rate, or make enemies take more hits, and I would say you got a good game here. Keep on going at it, you got potential.