This movie was shot in Spain and needed hundreds of Chinese extras, and the company sent scouts throughout Spain and the rest of Europe to hire as many Asian-looking actors and actresses that they could find. The casting web in 1962 reached as far as London, Lyon, and Marseilles, so the result was that many Chinese restaurants in those cities closed for the summer 1962 during filming because the restaurant staff - often including the restaurant's owners - was hired away by the movie company. The company hired so many, that for several months, there was scarcely a Chinese restaurant to be found open in Spain and those three other cities.
Charlton Heston's character, Major Matt Lewis, was loosely based on the real-life officer in charge of the Marine guard at the U.S. Legation, then Captain, later Lieutenant General, John Twiggs Myers U.S.M.C. (1871-1952). Myers was in the forefront of some of the toughest fighting in the besieged legations, wounded during the fighting on the Tartar Wall. Myers also later commanded the Marine Corps forces in Tangier during the Pedicaris Incident, and as such was portrayed as Captain Jerome by Steve Kanaly in The Wind and the Lion (1975).
During the opening scene, where the Legations and their flags are displayed, the camera lingers on the Spanish flag for a few seconds. Given the fact the Spanish legation or diplomats are not mentioned in this movie, some have commented the flag was only shown as this movie was filmed in Spain during Francisco Franco's regime. While this explanation probably has some truth, there was in fact a Spanish legation and diplomatic staff in Peking during the siege. However, like the Dutch and Belgian Legations, they had no Marines and played little role in the siege. This is why "eleven foreign powers" are referenced, but only eight nations are named.
Charlton Heston later recalled that during filming, his chauffeur was not being paid by the production company. Heston twice confronted one of the producers about this and twice he got a very elaborate and flamboyant show of outraged indignation from the producer screaming over the phone at someone else, but the chauffeur was still not paid for his work. This annoyed Heston so much, that he personally paid the chauffeur, and when he confronted the producer over it, the producer then handed over enough money to reimburse Heston.
In China, at the time, working as a professional mining engineer, future President of the United States Herbert Hoover and his wife were civilians under siege at the foreign legations' compound. The future first Lady, Louise "Lou" Henry Hoover, collected shrapnel from Boxer artillery that is on display at the Presidential Library in West Branch, Iowa. The Hoovers picked up Mandarin Chinese while in China and used it at the White House when they didn't want to be overheard.