On the Nation of Taverix
Taverix is something of an anomaly amongst the countless Celtic tribes in Europe. While most remained fairly primitive and disorganized, Taverix became a single nation.
It was the result of an ancient Chief, the eponymous Chief Taverix, who first united his region by the sword. Appointing his most loyal followers as Chiefs in their own settlements, he then declared himself to be 'High Chief'. He has since passed by, and new High Chiefs are chosen through the agreement of the lower Chiefs.
In Taverix, the High Chief holds authority, but Chiefs have a very large degree of independence. People have great loyalty to their Chiefs, often much more than to their High Chief. Unity is ensured by the fact that attacking one of your fellow chiefs is likely to result in your annihilation at the hands of all other chiefs.
The result of this is that the best way for Chiefs to rise in power, possibly someday becoming High Chief (or sometimes becoming completely independent), is to expand and raid outside of your borders. There is also a good degree of internal competition and downright infighting, which the High Chief is required to frequently crack down upon.
The end result of this is that Taverix expands at an extremely fast rate, and raids outside of its borders constantly. Different chiefs often work incoherently in their efforts, sometimes fighting each other to control a new area. Taverix is thus prone to splintering and disorder- but none the weaker for it.