Some ideas
Great game, I beyt that if you made a sequal it would do really well. Here are some of my recomendations.
>Difficulty settings
>More enemies: It got really easy after i got a lot of the big turrets.
>Challenge modes: One idea I had is that you start off with a certain amount of $, and you have to survive with it as long as possable. That way you could spend it all right away, but then when your turrets run out of ammo you will lose. The stratagy would be to only make as many turrets as you need to survive, and to be conservative with ammo.
>Scores and a high score bord: That way people will be able to show off how good they are.
Those are just a few of the ideas I had while playing. Don't care if you use them or not.