I recently saw a documentary about the exalted princes of the ancient celts, and the thesis that was argued was that these princes resigned over a few large kingdoms which possibly predated the Phoenicians due to the presence of an vast over-land trade network. The thesis argued that the evidence presented by the archeological findings suggest that a vast trade network existed which was overcome by the much more vast Phoenician network, and that it is possible that the Celtic culture was much more peaceful in its much earlier days than what was described by both Julius Caesar and pre-Julian Roman sources. Is there any more information that we know about this ancient civilization, and how far back can we go before our guesses outnumber our assurances?
What we know about Iron Age Gaul essentially comes from archeology and historical sources : unfortunately, the first is framentary and essentially focuses on what was preserved of material culture and interpolating about what it implies for protohistoric societies and immaterial culture; while the seconds were written by Greek or Roman authors that while not necessarily being hostile, wrote from an outsider point of view.
The further ancient sources goes are mentions of the Ligyes (Ligurians) living in the Mediterranean shores of Gaul, as a set of people trading and interacting with Greeks in particular. Before that, we're entirely reliant on archeology for direct elements : we know that Late Bronze Age and First Iron Age Gaul was part of a broad exchange network centered on the Mediteranean basin, but Phoenicians weren't really part of it except in the region set between Pyrénées and Hérault river, where a probably Celtic-speaking population got Iberized along Puno-Iberic exchanges : most of phoenician products in Gaul probably got acquired trough these regions, with Iberian/Iberized peoples as intermediaries. The idea that Phoenicians had a strong presence in Atlantic Gaul is simply not corroborated by archeological evidence, no matter how ingrained it had been in XIXth and XXth regionalist historiography (Carthaginian coins found in Brittany could well have been coming from Africa....carried over by a soldier returning from the conquest of Algiers, other evidence being forged). Contrary to the set of elements pointing at a possible seasonal presence of Carthaginian traders in England, these seem to have bypassed Gaul.
Eventually, before the VIth century BCE and the first historical mention of the populations of Iron Age Gaul and the trade monopoly of Ligustikè/Keltikè shores by Greeks (at least between the Var and Hérault rivers) the best evidenced trade in the region is Etruscan, to the point Mediterranean Gaul is the place where most Etrsucan artifacts were found outside Tuscany, possibly out of contact within the Urnfield Culture, a Bronze Age archeological culture spanning from southern Gaul and Italy to Denmark, predecessor of Halsstatt and LaTenian cultures and largely including (if most probably not exclusively neither integrally) Celtic-speaking populations. These exchanges really boomed by the VIIth century BCE, with a first development of proto-urban centers along the Ligustikè (such as Pech-Maho, Saint-Blaise, Naro, Lattara) allowing Mediterranean traders to find commercial partners, further followed by the VIth and Vth centuries BCE by a first agglomeration dynamics beyond the Mediterranean arc (Avaricum, Huneburg, etc.) both hinting at some centralization of power (notably around sanctuaries).
Is it enough to think of them as "few large kingdoms"? I admit I'm really puzzled by this, because it would arguably refer to a misunderstood or faulty interpretation of the Hallsttatian/early LaTenian "princeley seats" or Fürstensitze
Historical mentions about the First Iron Age are really scarce apart from a probable Gaulish tradition recorded by Livy, accounting for Ambigatos being invested of the imperium over Gaul by being king of the Bituriges, organized the migration of Gaulish peoples in Italy. Employed lexicon is on line with the particular form of "high-kingship"/"high-federal" organization of Gaulish petty-states of the late independent period; but the text is quite problematic in other aspect (notably messing up datation of events) even if we'd agree representative of the Gaulish political structures of the Vth century BCE (which is debatable, as it might be a partial re-make on more recent political lines) we'd be talking of a federation of federations, rather than a kingdom (comparable to, say, early medieval Britain, Pictland or Ireland).
Large polities usually leave traces of their existence : coinage, monuments, widespread social differentiation, warfare, etc. We could have no written evidence on Romans, Hellenistic kingdoms, etc. and still spot something.
Archeologically, what would fit "best" would be the "Hallstattian princes", whose ostentatious chariot/enclosure burials on the northern Alpine arc (such as Vix, Hochdorf, Lavau, Glauberg, etc.) and "Princeley seats" sometimes associated with graves (Vix or Glauberg), sometimes not (Zavist, Avaricum, etc.), sometimes with proto-agglomerations, sometimes not. While there's still some debate about the emergence and the relation of both phenomenons, it seems that as it happened in Mediterranean Gaul, first centralization and early social differentiation (on a still largely more or less egalitarian basis) preceded access to Mediterranean trade, maybe out of a demographic and productive growth (having favored access to trade, rather than the reverse dynamic).
But not only these elements pre-existed Mediterranean trade or were "fueled" by it rather than being hijacked, is it enough to make them "few large kingdoms"?
The heterogeneity of these social development could be explained by the constitution of "federated" religious centers and sanctuaries where forms of centrality could take place, with inner and eventually external factors favoring their lasting existence. What we know of Mediterranean Gaul seems to corroborate this, with both the formation of "oppida" around sanctuaries but as well the formation of local federations as Elysices or, hypothetically, "Keltoi" whose fame or success would have led to expand/regionalize into a macro-regional meaning.
While these "princes" were certainly powerful (although without much separating them from a broad warring-aristocracy), their control and seating on trade roads and immediate regional population neither implied or required a big territorial spawn.
I realize it might be a pretty much frustrating answer, but having not seen this documentary (or having no idea which documentary you saw, truth to be told), I'm not really well set to say how much it's right or wrong even if what you wrote about it leaves me extremely skeptical. What's it's name?