This time nothing Twitch Prime, so no possible protest about the fact that the game is not "completely free": The offer comes directly from Steam and let's talk about Crusader Kings 2.
Developed by Paradox Interactive (when you name Paradox there is a Fraws in raptures for Cities: Skyline), the game released in the 2012 was the excellent result of Crusader Kings.
The game is a medieval strategic simulator in which instead of having in hand the fate of a nation, we have a dynasty to control and is set by the year 1066, with the ascent to the throne of Guglielmo the Conqueror, until at 1453, fall of the Byzantine Empire.
The title provided is only the basic edition, but with the DLC you can start our adventure from 867 with Old Gods or even from the 769 with Charlemagne. Particularity of Crusader Kings 2 (like its predecessor) is to be able to export the game in Europe Universalis IV, so as to continue our game for about another 400 years.
Unfortunately the offer is really short: you will have time to download the game from this link until the 7 April.