I decided to come back and crush this game after thinking about pattern options. Getting an all-time high score without juggling wasn't difficult once I understood how the game mechanics worked. (My fault for not reading the "How to" before.)
What SensitiveJerk said about lack of tower balance is definitely true. Green towers are by far the weakest- regardless of their level.
Two stars for a simple concept that had me thinking about it when I wasn't playing. One star for simple and clean graphics. Half a star for music (and lack of attack sounds) that didn't drive me nuts or give me a headache.
Minus two stars for either not living up to potential, or obscuring detail. By that I mean that there wasn't an explanation of the strengths/weaknesses of enemy units, there didn't seem to be any useful bonuses for clearing/sending waves as quickly as possible, and four tower mode didn't seem to provide any greatly increased challenge.
Good work. :)