Mount & Blade
 

Its this one Indie game on steam that's extremely fun. its marked off by 85% for this weekend, so download it now if you wanna give it a try.

Its a pretty realistic middle-ages simulator; no magic or any of that, just combat, strategy, and class system tension to take into account.

It's a sand box game too; theres no linear story line. you can do whatever you want, align whoever you want, or just make your own damn kingdom.

Whoops! Forgot the link.

here it is:

http://store.steampowered.com/app/22100/