"Celt" Conjecture

[MD]

These people may very well be the same that inhabited the Oppida. Later (BG VI 24) he mentions that Gauls (Volcae Tectosages) are still living east of the Rhine. Thus there isn't any real contradiction between Caesars writings and archeology.

However archaeology does not show where the Gauls lived, it only gives an image of the material culture.

Let's read BG VI 24: ... now also they continue in the same scarcity, indigence, hardihood, as the Germans, and use the same food and dress; but their proximity to the Province and knowledge of commodities from countries beyond the sea supplies to the Gauls many things tending to luxury as well as civilization.

Well archeology tells us the material culture of these people that inhabited southern Germany and built hillforts and later Oppida belonged to the Hallstatt and La Tène cultures today commonly associated with Celts. That there was a gradual shift from Hallstatt to La Tène phase. In the 3rd and 2nd century BC they built proto urban Oppida and begun minting coins. Evidently they had contact and traded with the Mediterranean civilization as early as the 6th century BC. For example sherds of imported Greek pottery was excavated at several places like Würzburg.
At the same time those people that inhabited northern Germany had a distinct material culture, today called Jastorf culture and commonly associated with the early and proto Germanic tribes.
Than in the first century BC, when coincidently classic authors tell us the Celts were emigrating from those regions because of pressure from Germanic invaders, the Oppida were abandoned and there coinage disappears. Settlements change and the material culture with tem, now belonging to the northern type. Since this happened over several decades there's no surprise to find mixed populations and increased cultural exchange.

Maybe the oppida builders were Germanic and just disguised as Celts until the Roman conquest robbed them a reason to do so, but maybe the ancient authors just told us a little bit of the truth. Since it not just one author like Caesar, who pursued his own agenda, but others like Tacitus or Claudius Ptolemaios who tell us the same basic story you can't just explain it away.