Bore da, Steve.
The ancient Celtic peoples are credited with the invention of what we know as soap, they were advanced metalworkers and the Latin, i.e. Roman, words gladius and carrus are ultimately derived from Celtic languages. The ancient Celts, along with the Etruscans, developed chain mail and I believe the Roman helmet was also based on a Gaulish model. The Gauls gave the Romans such a hiding at the Battle of the Caudine Forks, where they forced the Romans to march “under the yoke”, that it led to a complete restructuring of the Roman military along more advanced Greek lines. We also know that various Celtic peoples had walled cities and writing was used, albeit sparingly. The amount of trade that they did with the Mediterranean world, also suggests they had plenty of goods and materials that were needed or desired elsewhere.
The Celtic peoples lacked the centralisation of other populations and that was, in my opinion, the fundamental difference and, perhaps, weakness in the face of the Romans, but they were nothing like the stereotypical barbarians on TV either.
Mwynha dy ddiwrnod!