Oppida were not city centers: they were hill forts. An encampment replacement makes perfect sense.
"oppidum" is a Latin word and "city in the countryside" every new pupil knows that.
Julius Caesar used it to designate the Celtic hilltop settlement. Most of them were refuge. This kind of places exist every were in ancient Europe (also on Crete) and is nothing special for the Celts. They were mainly used in times of war.
I remain, military training of any kind, even if they were very warlike, and +1 housing does not fit to the Celts. Military experience was more a tradition and men ritual. They were not particularly good at defense. Technically Caesar just needs two years (58-56 BC): "Veni, vidi, vici".
And finally, how do you want to bring the Druid, which starts with the ability to recruit adjacent barbarians, then?
How do you want to bring the UU, which is invisible in forests and the headhunter promotion?
It is difficult to bring all aspects of the rich Celtic culture together and give them a unique gameplay (more than just distributed bonuses). What sort of victory would the Celts prefer? Try it!
Look, if we're going to base this civ on Roman propaganda, we might as well throw in the human sacrifices.
The Celts were not barbarians; they were an urban culture who possessed advanced craftsmanship (that excelled that of the Romans, I might add) and rich traditions of poetry. That they were less urbanized than Rome (one of the most urbanized civilizations in the world at the time) doesn't make them hut-dwelling cavemen.
The Celts moved around and robbed gold like Conquistador and they had the best weapons of their time.
When the Romans called them barbarians, the first thing they ant to say was that they did not speak Latin or Greek language.
Robbery and piracy were not special in this time even Odysseus was a pirate and the Roman history included the "rape of the Sabine women".
The winners write history. Too bad that the Celts had only oral traditions. So the most things we know about them is from their enemies written. By the way, what others have written about the Minoans was similar, if I think of the labyrinth, the Minotaur and Icarus. But later the memory of Minos was very positive. The conquerors saw that they had learned a lot from the Minoans.
There are a lot of archaeological evidence of human sacrifice and cannibalism among the Celts. But that is probably a cultural stage, if I think of Abraham of the Bible, Homer and Carthage.
But unfortunately, as I wrote, when I made my suggestion for the Celts, was, that nothing came to my mind about bardic songs. So a small culture boost could be ok. That could help them, when they not build theater districts.
The problem is that Brennus is only a name. That might be fine for Civ3 or Civ4, but in Civ6, where the leaders are chosen for their personality, that rules out someone who is no more than a name. I'd also point out that when Roman historians talk about the Celts, they're wrong more often than they're right. Likewise for the Carthaginians. Or anyone else they regarded as a blood enemy. The Romans were excellent propagandists; Kim Jung Oon has nothing on them.
Ah, like our informations about Vercingetorix.
We know this one over 2000 years old sentence of Brennus, tells us very much.
He was a tough and harsh guy. He was carrying weapons. He united a huge band of robbers. He thought economically and foresighted. He weighed well his chances and the attendant expense. He just want to sack and the gold and then return home nothing more. He knew about weights. He could calculate. He was clever and tricky...
That fits to my suggestion for a Celtic Civ. Here again this time with a small culture boost for the bardic songs.
LH: Brennus (speaks and looks Gaulish)
LUA: "Vae victis!": If he conquer a city he can choose not only to conquer or raze it, he can also choose "Vae victis!". Then all buildings, districts and improvements of this city get plundered and Brennus gets a huge sum of gold.
But the city does not change the owner. In addition a forced cease-fire will enter into force for 10 turns (had to be tested or discussed). Or, it will be a forced peace deal and after some turns the victim gets a casus belli for revenge.
(This ability is supposed to represent the recurrent invasions of the Celts. They were not seeking for new places made of stone to live there, they were just seeking for free goods to go.)
Agenda:
He is always ready for war.
He always tries to conclude favorable peace treaties.
He does not like when his offers are rejected.
He did not like those Civs who try to take revenge.
He likes Civs who did not try to take revenge.
Civ name: "Celts"
Symbol: wild boar
City names of the La Tene culture should not be a problem. Ptolemy called enough oppida, so that neither modern names nor names from archaeological sites would have to be resorted to.
UA: Ferrum Noricum: the palace generates two iron resources for free;
all unit which require iron get +3 battle strength;
every iron, copper, resource provides a bonus +1 production and +1 science
every gold, silver resource provides a bonus +1 culture and +1 gold (for human sacrifice, bardic songs and trade system, goldsmith)
UU: Forest Devils: swordsman replacements which are invisible in forests or could only be seen from adjacent tiles.
They can move undisturbed in forests and see through them.
They get +2 for fortify on forest hills.
Also they get the "headhunter" promotion if they kill an enemy unit for the first time. With this promotion they deserve faith for kills.
UD:
Druid circle: holy district replacements which gets +1 faith from every adjacent forest instead of +1 for every two forests.
AND all apostle (with the look of a Druid) which are formed there starts with the ability to recruit adjacent barbarians (So this could lead the Celts to no longer develop their religion as many have noted here.).
AND all religious units formed there can embark without losing extra movement points AND can heal in forests tiles (Druid power drink).