Open source civilization
Freeciv is a Free and Open Source empire-building strategy game inspired by the history of human civilization. The game commences in prehistory and your mission is to lead your tribe from the Stone Age to the Space Age...
The Total War series can be a great way to teach history. Note that they're not historical accurate perse. But that can be the basis for a nice tlesson series. You can het the older titles for a bargain. Tip, Rome: Total War