I find it interesting that Celtic campaign has many similiartity to Roman. Most notably is the goverment building. Celtic are suprisingly have many building unlocked to them at even the lowest tier(protectorate) which means even settlement that you cannot make use militarily, Celtic can still utilize them economicly which is simliar to Roman province although they are better at it since they have no building restriction at all.

Second is manpower. Celtic can utilize large manpower but require their own culture to have access of such manpower which means territories outside of western tribal states the Celt would have to rely on Protectorate or Client state. Again similiar to Roman on how they handle province outside of Italy.

And finally both of their traits mechanic encourage expansion although how they get is diffrent. Celtic being more strict and have to do it in young age while praying you pass the Kingetos instead of Weak Kingetos but getting it is quicker and give stronger trait while Roman military command politicly restricted to older man with Preator or Consul trait but every Roman generals could become one as soon as they are eligeble for the election.

I find their similiarity to be interesting. Playing Celtic is like playing Roman before it become the juggernaut it is in the campaign since you need to expand Celtic power base first.