Regional allied tributary

[BiggBudd]

Hi peeps, having trouble building this, is it only available in foreign lands? What is the tech tree to build it?
ps am playing as britions provin campaign.

[Zarax]

allied government is not available in your factional homelands as that would cripple recruitment.
Outside celtic areas you need to destroy the current factional government (if it's a barbarian area) and build military occupation.
After that, regional allied tributary will become available to you.

[BiggBudd]

What would I do without you Zarax!

[Zarax]

BTW, before choosing the government type I'd look for what aux barracks give in the building browser, not much of a point in going allied if what you get is not useful...