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Unlock all factions
Find this directory:
Medieval II Total Wardataworldmapscampaignimperial_campaign
Then find the file descr_strat. Open it with a text editor (Word, or somthing like that). Cut and Paste the 'unlockable' section into the 'playable' section and save the file. Launch the game and you should have every faction available.
You can also unlock the Papal Estates, but not the Mongols, Timurids and The Aztecs.
Remember to make BACKUPS before you are editing the game files!!!!