I’m reading an interesting book called Discovery of Middle Earth by Graham Robb.
Its a story of the celts. The book ties them to the ancient greeks. their gods were very similiar. Hercules was central to them. the druids were not just a blood sacrifice cult. they were also hugely scientific. their astronomical formulas were much similar to the hats mentioned here. julius caesar was able to easily make his way through gaul and england because the roads were all already neatly laid out.
the interesting thing here is that this books dates the celtic civilization to only about 600 bc. they were invaders who mixed with the locals. (unlike the germans who had lived in the forests for a long time.)
these discoveries make it sound like the locals were already pretty advanced when the celts arrived
It's true that the Celts entered Europe from somewhere in central Asia no earlier than 600 BC, but there were no Germans already living there. It's not unlikely that the pre-Celtic 'coat of paint' was driven out (perhaps 'squirting' into the lower Danube basin) or just killed off, or may have been an earlier drift of Celts anyway (P-Celts first, Q-Celts second). The Germanic peoples spread down from Scandinavia, having been there for more than a 1000 years.
There was no "battle that stopped Rome" (the Varian disaster), and the Romans continued to spread east of the Rhine, eventually building a settlement (and likely a fortress) at what is now Copenhagen, and completely depopulationg the penisular area leading to it. When the Huns came in during the Dark Ages, they displaced the Germanic tribes which had spread furthest east, and another group came out of central and/or northern Asia, the Slavs.
The Franks were Germanic tribes who moved into Gaul during the last century or so of the western Roman Empire, under pressure from the Saxons. The Saxons also wound up conquering and settling Britain and moving into the former Roman territory still known as Saxony. The tale of Beowulf references places and probably legendary people from what is now Denmark.
Julius Caesar took his army throughout Gaul, but also built temporary but very impressive bridges over the Rhine, crossed over, kicked a few butts, then after some days of marching, built a second bridge to return to Gaul. Both bridges were dismantled after use. There's no surviving sign the Gauls built any form of bridge. When he got to Britain (twice) he picked areas to land where the interior could be reconn'ed from offshore. The Celts used chariots in both Gaul and Britain, but the Romans knew how to cope with those, as the Celts found out. There was already Roman contact with various tribes in Britain prior to Caesar's short campaigns, and some British tribes coined their own money.
There's been interest in the pre-Roman roads of Britain and western Europe, but whatever was there may or may not have been extensive and regardless was of very uneven quality. I suspect the existence of the road networks was a consequence of the cultural and linguistic commonality of the neighboring towns to faciliitate trade in local markets, and it did make it easier for the Romans to find the oppida and conquer them, but I think there's been a great deal of oversell on that. It's easy to believe the roads made it easier on itinerant musicians and other performers, and probably these hat-wearing guys who may have also been what passed for the health care system.