Opening scene - A bit of history-

They were Germanic tribes if I remember correctly.

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The movie showed the end of the Marcomannic Wars. The main purpose of that 14 year long series of wars against a dozen or more Germanic tribes, was to whip them into submission.

The Northern European tribes had grown bold, started making alliances and even invaded the Empire, making their way across the Danube (the border), slaying 20K Roman soldiers in a huge battle, then even getting as far as where Venice is now.

This was the first time Italy had been invaded in 300 years. The Romans won, of course, but it was just the first of a long line of 'barbaric' invasions that would ultimately be the end of the Empire. Or as Wikipedia phrases it: "The Germanic tribes were temporarily checked, but the Marcomannic Wars were only the prelude of the invasions that would eventually disassemble and end the Western Roman Empire in the 4th and 5th centuries."

https://en.wikipedia.org/wiki/Marcomannic_Wars

If you are ever in Austria you can visit the garrison that the Romans had near the battlefield that took place in the film: "The Ruins of Carnuntum"
Maximus´servant mentions Vindobona to Lucilla (now known as Vienna) you can get there from Carnuntum following the "Limes" built by the romans (now Simmeringer Hauptstrasse)