Too much is being made of too little. The Celts were widely spread and
the name applied to a series of peoples that lasted a thousand years or
more, all of whom were going thru changes.

The Celtic Hill forts, for example first show up 5000 years ago, but
then a few hundred years later were abandoned. This is a repeating cycle
seen in many areas. then too, one region would be building hill forts
while another abandoned them.

Partly this was due to the land being played out, so people pulled up
stakes and pioneered in tiny cabins hidden away in the bush, such as
outlined by Barford in "The Early Slavs". The name is oxymoronic. The
Slav was so skilled at living off the wild food in the forest, that like
Native Americans, they would simply run off, and could not be enslaved.

Then too, all the precise order of classes in this thread was repeatedly
wiped out when a pandemic broke out, and the survivors, no matter how
poor they had been, became aristocracy.

Its well documented how Bubonic plague created such a labor shortage
that any competent man could name his price. To be sure, this kind of
thing happened in earlier epidemics. Then too, as Machiavelli pointed
out in the city states of Germany, a craftsman could, and did, vote with
his feet to wherever the taxes and cost of management was lower.

All these elegant designs you read about were written by the scribes in
the pay of the warrior class and are the same kind of propaganda about
good order that we still see.