Good implementation of the theme -- I like that it doubles your current money, giving the player a meta risk strategy of not buying upgrades for a few rounds together up a greater total to double later.
On to the combat itself: I would have liked to be able to attack in the direction of my mouse, rather than in the direction the character is facing, since most of the gameplay involves strafing the enemies, going backwards. Which meant that I keep having to walk towards the enemies in order to hit them. And since, in the first (and second?) wave, the big guys kill me instantaneously, so it's very important to stay far away.
This basically led to a fairly tedious optimal strategy of walking in big circles and chipping away at their health over time. I think a longer attack range and mouse-direction attacks would make this feel a lot more manageable.
Also, the big guys maybe have a bit too much health? It feels like they take too long to take down; especially given that they're almost guaranteed to be killed last and have a very simple strategy, leaving the player with a fairly tedious job of chopping them down one inch at a time.