As Zomby_Woof says, you can make factions playable by editing desc_strat.txt, which will only take a few seconds. Remember to back-up your original files before editing though, saves reinstalling to get hold of the originals
However, I'm sure v1.3 will give a crash to desktop (CTD) because there are no campaign descriptions present for Armenia, Spain, Numidia, Dacia, Scythia, Thrace. These are found @ C:\Program Files\Activision\Rome - Total War\Data\text if you want to create your own.
You can always download a mod which does this for you, try Borsook's all faction mod for RTW v1.3 / BI v1.4 available here.
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