Barbarian Reforms

Anyone know the criteria to get them? I am playing as Casse right now and got Time Of Bondsmen and I am not sure exactly how I got it.. though I know I got it around 30 years too early according to the pop up. I will say this though... this is my first "Barbarian Campaign" and playing as Casse I think is a perfect introduction to get use to your men *though it seems in every Total War game that who ever has the English Isles has an unfair advantage just do to the richness of resources and total lack of enemy neighbors*.

They are in the FAQ:
https://forums.totalwar.org/vb/showthread.php?t=84854
They are pretty easy to get, since all Celtic factions are contributing.

Thank you... also I seemed to have posted this in the wrong area sorry admins.