On my first serious (as in I sort-of knew what the hell I was doing) game as Count of Desmond, managing to crown myself Emperor of Britannia and think that things would work well from there onwards only to gasp and stare at the map horrified as I saw the ENTIRETY of France succumb to an Almoravid Invasion (which I might have helped by being at war with them at the same time for Normandy, but... yeah), making it the supreme muslin power by controlling both Francia, Hispania, Mauretania and half of Africa. Then, seeing as Christianity would probably be doomed (the HRE was too busy playing Whack-a-Pole while the Byzantines were having trouble with the arabs and Ilkhanate and the Golden Horde was inching closer and closer to the german borders and Sweden), I took Britannia on a massive blitzkrieging, truce-breaking, assassinating, maiming and bribing campaig while pushing the Moors back, from Flanders all the way back to Mali, duchy by duchy, growing into the most powerful nation on Europe by a wide margin, with the only other independent nation west of the HRE being Navarra (!) on a small patch of land in the middle of France (!!!) and, strangely enough, the King of France. Which ruled a single island off the coast of Flanders and NOTHING MORE.
And then I conquered Egypt.