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.