There are some crucial battles that determine the future of certain factions, the main ones being Almohads v Spanish, English v French and Byzantium v Turks.

The victors in these wars can vary every single game, sometimes a new faction joins in as a minor power, Aragon, HRE, Hungary and Egypt being the ones most likely to intervene and turn the tide of a war.

Sometimes I see the Spanish conquer the Almohads in no time, sometimes I see the Almohads become a major superpower dominating all of the west of the map. Sometimes the French do the same etc etc etc.

It varies all the time which is why there is so much replay value.

BTW the Aragonese are nowhere near as hard as they look, just backstab the Spanish when the time is right and you can replace them and continue their war with the Almohads. Then just pick specific provinces for conquest, Sicily, Sardinia/Corsica and Malta are good. Sicily for trade the others purely for strategic value this way you will dominate the central med and therefore all of the trade. By the way, since so many provinces trade in silk in the west, you need to conquer provinces that dont have just silk or ones that have a few tradable goods, otherwise you wont make to much money. A strong navy is vital.