Crusader Kings II is a strategy game available on Linux, featuring single and multi-player, and set in various historical periods. Crusader Kings II was launched in 2012 and it received positive reviews.
(Image credit: http://store.steampowered.com/app/203770)
Currently, there is a 75% off sale on Steam, so you can buy the game for 9,99€ until October 6. Furthermore, Crusader Kings II also offers a free 2-day trial so you can install and play the full game for two days, until October 6.