How to Unlock all Factions in Medieval War 2 Total War for Steam
READ
Medieval II Total War\data\world\maps\campaign\imperial_campaign
Find the file descr_strat open it with a text editor or something similar. Cut and paste the unlockable section into the playable section and save the file. Launch the game and you should have every faction besides papal states,mongols,aztecs, and timurids.
You can get the papal states but not the Mongols, Aztecs, or Timurids