Just the right amount of juice, I love how it looks! A few things that I think could be improved:
Make the enemies more distinguishable instead of just making them bigger or smaller. When I first saw the small enemy in the circle above I thought it was the same enemy as before (the medium sized one) so I didn't prepare accordingly;
You could add a small area at the top where you can't place any tower; when you first play the game you don't know that different enemies go at different speeds. By blocking the top area the game prevents you from placing all the towers in the top row (like I did) and almost having a heart attack when all those little enemies slipped by.
The levels are quite long, it would be nice to have a sort of progress bar that tell you how many waves you still have to survive to.
;)