The film was originally intended to be made after El Cid (1961) and to reunite Charlton Heston and Sophia Loren. The set for the Forum Romanum was being built when Heston rejected the script and expressed an interest in 55 Days at Peking (1963) instead. (Heston got on very badly with Loren on that film and refused to work with her again.) Samuel Bronston immediately ordered that the work on the Forum be stopped and the landscaping and foundation work be adapted for the Peking set. After filming, the Peking set was torn down and replaced by the Forum. If you look carefully, both sets share a very similar topography.
On the flight to Spain, one of the film's writers struck up a conversation with Alec Guinness after seeing him working with the script; the actor stated that he disliked his lines and was rewriting them before starting memorization.
The 'Battle of the Four Armies' involved 8,000 soldiers including 1,200 cavalry and was shot on an undulating plain at Manzanares el Real which allowed large numbers of soldiers to be visible over a long distance.
In later years, Miramax would acquire the US distribution rights to the film. After the founders Bob Weinstein and Harvey Weinstein split with Miramax parent Disney, they formed the Weinstein Company, who currently owns US distribution rights.
Anthony Mann filmed for 143 days, while Yakima Canutt and Andrew Marton filmed the action scenes simultaneously in 69 days. Filmed in Rome and Madrid, this was one of the most expensive pictures of the 60's costing $16m, or an unofficial $20m.
Christopher Plummer was amazed at the lavishness and no-expense-spared aspect of the production, a Rolls Royce was at his disposal for the entire duration of the shooting. This was a far cry from Samuel Bronson's previous "King of Kings" where when Jeffrey Hunter and Robert Ryan's car broke down on the way to the sermon on the mount scene, they had to push it to get it started .... In costume!