The symbol Timonides wears around his neck is a staurogram, an early symbol of Christianity. It's the only mention of Christianity in this movie.
Richard Harris was originally cast as Commodus. He withdrew because of artistic differences with director Anthony Mann. He was cast as Commodus' father Marcus Aurelius in Gladiator (2000). *However, the following year the two men worked together on the WW2 thriller, The Heroes of Telemark.
Sophia Loren's salary for this movie was $1 million. She was the second actress, behind Elizabeth Taylor, to receive that amount for a single movie.
Director Anthony Mann filmed for 143 days, while second-unit directors Yakima Canutt and Andrew Marton shot the action scenes simultaneously for 69 days. Filmed in Rome and Madrid, this was one of the most expensive pictures of the 1960s, costing $16 million, or an unofficial $20 million.