Liked the art, very humble but gets the point across  :P

• Check the collision on the characters, sometimes the bullets just went across the little dudes
• After 3 guards the game gets broken and there is no difficulty, I just end up watching it work
• It wasn't clear that I need 50bucks to buy guards, you can add a blinking red text to warn the player

Overall great idea, my advice is to add a tower defense mechanic, that would rebalance the game.

Keep it going brotherman