Imagine if you combine missile command with any desktop tower defense game, and then add upgrades,, with some nice graphic touches, thats Revenge of the titans for the PC.
Essentially you construct and command your ground defences in a series of increasingly massive battles across the solar system, in their 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!
It was released exclusively on the Steam platform, and can be downloaded and installed for less than 15 USD.
I was quite impressed, I have a soft spot for tower defense games, and at less than 15 dollars its a must buy in my opinion, check out the video to get an idea of its unique faster pace, and its graphic charms.
256 MB RAM