Is there multiple upgrades for the tower?
i.e. a basic cannon can become upgraded to a better version or like a AA gun
Absolutely love this game! I have the pro version. Levels 1 thru 8 were fairly simple. I must be missing something because level 9 is killing me. I get to round 20 and lose every time. I usually start the round with 15 - 20 lives. Any suggestions?