The Real Time Strategy game franchise, Age of Empires, has tens of millions of games in its history, however the latest release of this PC game is free to play.
Players take control of various historical civilizations such as ancient Egypt or Greece and then embark on quests to lead each civilization through the ages, improving economy, technology and it’s military might.
The single player experience is quite addictive but once you achieve a certain level of experiece in the game you’ll be able to participate in larger scale online game modes.
As I mentioned Age of Empires Online is free to play, however premium content will also be made available at a cost. Get more information at the Games for Windows website