The gameplay is really well done! I liked the fact how the levels are tied to different upgrades---although I sticked to regen mainly. It is overall really fun :D
One thing I really liked was how the bullets are placed around the player. Along with the polishing, it feels really well implemented.
A small thing I found out is that some enemies seems to drop a different amounts of money, making the money pay of each level a bit unequal.
Great job!