What determines stone forts or wooden forts' creation

[SizarieldoR]

I very rarely build forts and I always thought they were wooden. Then, I was playing with the Turks yesterday (on SS, Late camp.) and when I started to fight I saw that my fort was actually stone and had nice walls where my archers could freely fire on the enemy. Really nice surprise

Last I used a fort was in the vanilla and it was wooden. Or, it may have been in the early campaign. I was wondering how do you know if you've built a wooden fort or the much cooler stone fort when you play your campaign

[Vissewalde rex de Gerzika]

I am not sure, but I think that SS has changed the wooden fort found in vanilla to the stone one you see in Britain expansion.

[Navajo Joe]

I can confirm that the Stone Forts from Britannia Kingdoms campaign was included from 6.3 onwards