From the developers page:
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!
Construct and command your ground defences in a series of increasingly massive battles across the solar system, in our frenetic arcade mash-up of Real Time Strategy and Tower Defence!
Take a visit to R&D to discover the latest technology, then build a network of blaster turrets, rocket launchers and laser cannon, explosives, barricades and shield generators, refineries and reactors, and send forth your own droid army to battle the invading alien forces!