Robo Defense is a Tower Defense game for Google Android Devices by Lupis Labs Software. Basically, you buy and upgrade defensive towers which you place on a map to impede and destroy invading hordes. As you destroy more enemies you gain more money to buy defenses. In Robo Defense, it is robots versus robots. The trick behind the game is to create a pathway for your enemies to follow which places them in under the heaviest concentration of fire from your defensive positions. Succeeding waves of enemies and higher levels make it more and more challenging as you go along.
The concept behind a tower defense game is simple, it is like a real time strategy game where you are purely on the defensive. Robo Defense was so good, and so addicting, I had to uninstall it from my mobile phone... well I am keeping it on my Wife's tablet, that way I can play it without having it with me the whole day. This in the interest of getting work done.
You can try the free version which can be found at this link: Robo Defense Free.
There is a paid version with more maps, levels and upgrades, which is well worth the asking price at this link: Robo Defense.
The free version will keep you entertained till the wee hours in the morning. The paid version, is something you best purchase before a weekend or when you are about to start a long vacation.