Savegames from 1.0 will not be compatible with 1.5. So you will need to start a new campaign. As far as unlocking the factions goes, it is simple.
Go to: RTW\World\Maps\Campaign\Imperial_Campaign\descr_strat.txt
Make a copy of it for a backup to be on the safe side. Then open it up. You will see the beginning looks like this:
Change it to look like this:;
Custom campaign script generated by Romans Campaign Map Editor
campaign imperial_campaign
playable
romans_julii
romans_brutii
romans_scipii
end
unlockable
egypt
seleucid
carthage
parthia
gauls
germans
britons
greek_cities
end
nonplayable
romans_senate
macedon
pontus
armenia
dacia
numidia
scythia
spain
thrace
slave
end
Do not change the rest of the file. Just the beginning section I have shown here.; Custom campaign script generated by Romans Campaign Map Editor
campaign imperial_campaign
playable
romans_julii
romans_brutii
romans_scipii
egypt
seleucid
carthage
parthia
gauls
germans
britons
greek_cities
macedon
pontus
armenia
dacia
numidia
scythia
spain
thrace
slave
end
unlockable
end
nonplayable
romans_senate
end
The interface in the campaign selection screen is only made to display 20 factions, so the rebel/slaves are not shown there even if you unlock them as above. If you want to play them you have to move one of the other factions to unlockable or nonplayable. I put the senate in nonplayable, but that is just a matter of personal preference. However, if you do choose to play as the Senate do not click on the Senate tab, otherwise your game will crash.
Bookmarks