https://en.m.wikipedia.org/wiki/Amber_RoadAll the oppidum were connected by roads. Romans did not invent roads.
The oldest wagon and wheel found in human history was found in Northern Germany. There were roads in central Europe before Rome even existed.
Germanic water management was far superior to that of Romans', the dykes built in the lowlands drained swampland and terraformed vast swathes of the European continent into arable farmland. By their own accord the Romans themselves were unable to till these fields: they lacked steel heavy plow technologily and did understand how Germanics were able to farm in swampland and sustain such large populations.
The same technology in addition to automated vertical windmill pumps is how the Dutch were able to reclaim vast swathes of land from the sea. One of the greatest pragmatic megaengineering projects still impressive even by modern standards.
We didn't even get started on steel and military tech (spoiler: Romans just ripped off Hallstatt technology, they didn't invent any of it and could not even produce military grade steel by themselves, they had to buy it from Noricum).