Crusader Kings II is utterly magnificent. It's still by far the best Grand Strategy game that Paradox have ever made, a brutal cutthroat game of thrones, bloodlines, fragile alliances and real people rather than numbers to crunch.
It's also frequently on sale for £7.49. If you missed the recent Steam weekend and GamersGate sals, never fear, because Bundle Stars are on point.
You can get all the DLC at a discount too. Ta, gooner!