Marcus Aurelius (according the Edward Gibbon and other historians) dealt the Empire a long-term blow when he broke with tradition by choosing his only surviving son, Commodus, to be his successor, rather than following the tradition of chosing the best man for the job and officially adopting him. To the consternation of his legions, Aurelius never chose a military commander over his own son. When you decide to abandon actual history at the very beginning of your story, the rest falls apart.
Secondly, Commodus was murdered by his concubine (who drugged his wine) and a wrestler (who strangled him) in his palace. In fact, it took a few days for everyone in Rome to come to finally believe that he was actually dead. HE WAS NOT KILLED in a single-handed combat with the commander of the army (either Stephen Boyd or Russell Crowe).
Third, there is no historical evidence that a group of barbarians were burned alive in the Roman forum, as this 1964 film depects. The screenwriter seems to have simply lost his grip on any sort of reality and went totally "Hollywood."
Samuel Bronson (the producer) spared no expence to actually build an exact replica of the Roman Forum (rather than do it digitally as in "Gladiator"), so the scenes shot on this set are truly spectacular. The set (built in Spain) was said to have stood intact for some years, even after Samuel Bronson Productions went bacnkrupt (over this very film). I have no idea if it's still standing.
Christopher Plummer is too old to play the actual Commodus, who was only a teenager when he ascended the throne. However, the script actually does justice to the spirit of the historical character of Commodus, and Plummer brings the man to vibrant life. Both Stephen Boyd (as the army commander) and Loren (as Commodus' sister) seem wooden and fail to establish any on- screen chemestry to their love-stared characters, although Loren's legendary beauty is well worth the price of admission.
Alec Guiness, James Mason, Anthony Quayle and Mel Ferrer all do an excellent job with their roles, although Omar Sharif has little to do since his scripted character is only one-dimensional.
Because of its over-all production values, and an appropriate and moving musical score, this becomes a satisfying, eye-popping, "they don't make them like this anymore" epic. It must be seen in the Widescreen format to do it justice.