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