Roger Corman has an unusual reputation. He has produced or directed a large number of "cheapie" movies--including this one, which was shot in Russia(per the DVD commentary, even the interior shots were done on soundstages in St. Petersberg).
Sometimes, one just doesn't feel like some sort of heavy-duty, call-to-action movie--say, like "Spartactus"(1963). On the other hand, if you feel like being entertained, "Arena"(2001), is likely to "hit the spot" very nicely. Just like a lot of Roger Corman's films.
The fact of the matter is that, surprisingly, the screenplay is well-crafted and not as goofy as you might think. The Romans did try to implement their customs wherever they conquered--including the contests of the gladiators as they were being performed in the Great Coliseum in Rome. And there was even a disastrous attempt to perform them in Greece(the Greeks rioted because of the carnage). Female gladiators--while unusual--were not unknown and some of them even appeared in "Gladiator" with Russell Crowe. A wooden, scaled-down clone of The Great Coliseum in Rome is not only possible, it is likely that there were hundreds of them in the forested frontier regions of the Empire.
A nice, fun movie for an evening's entertainment.
But you could say the same thing about other movies from Roger Corman, notably "The Undead"(1957) and "The Dunwich Horror"(1969). Incidentally, many of today's movies are just that--entertainment--and, IMO, no apologies are necessary.
Movies with "a message" are not--and should not be--the only alternative.