Basic TD

A basic TD game I had made for a school project. However I am working on another TD game for the fun of it and would like to see what changes people think I should do from this one to that one. Any code suggestions would be great. Also, thanks to Python PyGame Tower Defense. Without that project I dont think this game would be where it is now.

uses python 2.6 with pygame 1.9.1

known things to be changed for the other game:
-towers will be able to be clicked and dragged from the icons

Please, let me know of any changes you would like to see.