Age of Empires II

A fantastic RTS set in medieval times made by masters of the genre, Ensemble Studios. The player has control over a society and guides them through four different time periods or "ages". The game begins in the Dark Age, where very few buildings and units are available.

After a short time when requirements are met, the user gains the ability to advance their civilization beginning with the Feudal Age, where more upgrades, buildings, and units become available. The next age is the Castle Age, in which the powerful castle may be built, and used to produce powerful units including the unique unit of each civilization. Finally, the user can reach the Imperial Age, which is reminiscent of the early years of the Renaissance. Once the user has reached the Imperial Age, they gain access to all the upgrades, units, and buildings that may be built and researched for their specific civilization.

This text is from the Age of Empires II Wiki, found here: Age of Empires II Wiki
You can buy the game here: Amazon
If that's not your style, there are torrents out there...