Definitely one of the best tower defense games out there.
User Rating: 8 | Robo Defense AND
There's not a lot to say about Robo Defense, because it's really simplistic, but very well made. You have a map, and you have a gate that you're protecting, and enemies come in from the opposite side of the map and you have to build towers to guide their path and take them out before they enter your gate (a basic tower defense game). There's not much variety with tower types, but it doesn't really feel limiting, it's just a nice clean approach to a simple mechanic. You have gun towers, rocket towers, and slowdown towers. You can also upgrade these towers as you want. Rockets can become surface-to-air missiles, mortar towers, or just an upgraded rocket tower; gun towers can become anti-air turrets, flamethrowers, or just an upgraded gun tower; and slowdown towers can have upgrades, or turn into teleport towers (but that has to be unlocked).
Besides being extremely addicting and fun, one feature that Robo Defense has that is really awesome is the addition of achievements and rewards points. Every time you complete a map, you earn points based on bonuses, aliens killed, etc., and you can use these points to buy upgrades. You can increase the amount of money you start out with, how many lives you have, how fast the rockets reload, how long flames burn, a vast array of others as well.
The free version is the exact same gameplay, but with the paid version ($3), you get 4 new maps and increased difficulty options, along with more achievements available. I would definitely recommend this to anyone with a G1. It's fun, easy to learn, challenging if you want it to be, all around just great! 
