A few weeks ago i found this game in my local Gameshop. its not expensive and a nice little Game for all TowerDefence Lovers, like me;)
"At first they came from the skies. In the ensuing war, Earth was laid waste, but we had won...
But that was just the first strike - now the invasion has landed! Defeat the returning Titan horde in a series of epic ground battles, save the Earth, again, and command the counterstrike against their homeworld!"
You have to protect your buildings with towers, build up Refinerys to gain more income and after each level you have 1 techpoint.
Just 1 more Level, then i go to bed..;)