I like that even the FREE version lets you buy upgrades to your weaponry based on your score and awards, but you need to the paid version to really advance. It gives you a chance to get really familiar with the game. Robo Defense has a pretty good following and cheats advice can be found HERE. This game we splurged for and buoght a copy for each of our Android phones, and have a bit of a family competition going on.
This is definitely a game worth paying for, but FREE version gives you plenty of fun gaming! Robo Defense is a game that my whole family, aged 5 to 40, have found addictive.
Phone Tested On: Nexus One