First open file explorer which should be on the task bar of your computer. Then click on the drive in which you installed your game (for most people it is the C drive). After you clicked that scroll dow to "SteamLibrary", then click on it. Then click on "steamapps", then "common". This is where all your game files are located so then find "Rome Total War Gold" (also you will find rome total war alexander as well right next to rome total war gold, if you would like to play as all factions in alexander as well the same steps apply going foward). Next click on "Rome Total War Gold" and go to the search bar at the top right and search "descr_strat.txt" click the first one that shows up. After you will see the words playable, unlockable and unplayable and a bunch of factions listed under each one. Then copy and paste the unlockable and unplayble factions into the playable list and delete the extras left in unlockable and unplayable. Finally save the file and open Rome Total War, and you will be able to play as all factions :)

(i did not make this method to unlock all factions its not my original idea i'm just helping to spread the word about it)