hi! Are you going to take advice from someone who hasn't earned any money from games? Ok, then read on!
I have tried the first level and I think that overall the graphics are not bad. The big green and red faces for example look good enough, in my opinion.
The presentation of the UI could perhaps use a more graphical approach? Perhaps a font with bigger letters or a colourful background perhaps?
My biggest tip for you is: add more visual effects.
When a bullet hits an enemy, show something - give extra visual feedback.
When you click on a turret, add a visual cue that you have just clicked on that turret.
Other simple general examples: a platform character walks: add clouds of dust under his feet.
A gun shoots, show muzzle flash and a bit of screen shake perhaps.