BRENNUS takes various elements from Livy's history of the Gauls' attack on Rome, plus Plutarch's biography of Camillus, and cleverly remixes them to fashion a ripping yarn of an adventure movie. The banishment of the Roman general Camillus, the controversy over the spoils of Veii, the involvement of the Fabius brothers, the flight of the Vestal virgins from Rome, the barbarian who tugs the beard of a Roman elder, the honking of the sacred geese, and even Brennus's scornful "Woe to the vanquished!" as he heaps his sword onto the scales, are all from the historical record.
To be sure, the historical accounts are a bit more complicated and far less flattering to the Romans, but this variant is exactly the sort of heroic tale an ancient balladeer might have come up with to flatter his hosts at a Fabius family banquet. (Our hero is the dashing young Quintus Fabius, appealingly played by Tony Kendall.)
As for the barbarous Brennus, the more I see of Gordon Mitchell, the more I respect him as an actor. He makes a terrific villain; when he declares that he'll enter Rome with his new bride's head impaled on his sword, I believe it! Mitchell was equally impressive playing a tortured hero (see THE FURY OF ACHILLES and GIANT OF THE METROPOLIS).
No luck so far in finding this sword and sandal classic on DVD, alas. I managed to track down a VHS tape, but it was from a degraded full-screen print in black and white. I'd love to see this in full color and widescreen.