Not able to access all factions in skirmish mode

[Dobie]

When trying to do free-play skirmish I am only able to access necropolis, stronghold, and sanctuary and only 4 heroes (2 from necro and 1 from the other two.) Is game progression necessary to unlock the others? I've played a bit of the story and played around with some other things, but I haven't seen how they would be unlocked.

Thanks for any help guys ~ I'm not too familiar with the Heroes genre, it is my girlfriends game and I have no idea how to go about helping her.