I think you’ve missed the point. The Gundestrup cauldron shows exotic animals that the Thracian metalworkers, probably to order, put on a cauldron for a rich Celtic chieftain – this suggests a people with a far more extensive global view than is often suggested. La Tène artwork is some of the finest artwork of ancient Europe and shows a fusion of indigenous Celtic motifs with Greek and Etruscan influences that again suggest a much more cosmopolitan society. You’re not just talking about a bit of jewellery at all and the influences of La Tène artwork have survived down to this very day.
Caesar wrote about conquering hundreds of cities in Gaul, note the word cities. The Celtic oppidum was no mere fortress and going beyond La Tène to the Hallstatt era, again, you find the signs of quite a sophisticated culture. This is without even mentioning Celtic Spain and Portugal – check out the beautiful sauna constructions at Citania de Briteiros. And, in terms of cleanliness, the Celts have been credited with giving the Romans soap. It’s a bit difficult to level the accusation of poor hygiene at the people who may have invented soap, isn’t it?
The Gaulish druids are said to have written in Greek letters, there’s plenty of archaeological material to back that up too, and, of course, this was when the druids were still very powerful and present within Gaulish society. Finally, the Greek geographer Pytheas, who lived in the 4th century BCE, (paraphrased by Diodorus Siculus) commented on the “civilised” people of southwestern Britain, or Belerion, as he calls it – that’s long before the Roman conquest.
As I said before, you need to drop the stereotypical and outdated views that have, unfortunately, been so promoted by TV and films and, in many cases, poor research.