Charlton Heston wanted Jeanne Moreau for the role of Baroness Ivanoff.

Charlton Heston's character, Major Matt Lewis, was loosely based on the real-life officer in charge of the marine guard at the US Legation, then Captain, later Lieutenant General, John Twiggs Myers USMC (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.

Due to mainland China's hostility and isolation from the Western world, a full-scale 60-acre replication of Peking 1900 (sewers and all) was built in the plains outside Madrid, and Chinese/Asian extras were flown in from all over Europe to provide the local Peking (the old name of Beijing) citizenry. A number of costumes for the Royal Chinese Court (the Empress, Prince Tuan's, etc.) were authentic ones from Tzu Hsi's actual court. They were loaned by an illustrious Florentine family (which wished to stay anonymous) but was able to rescue them from the collapse of the dynasty right after the Boxer rebellion.

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 the movie, some have commented the flag is only shown as the film was filmed in Spain during 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.

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, IA. The Hoovers picked up Mandarin Chinese while in China and used it at the White House when they didn't want to be overheard.

It is still possible to walk around the area of Beijing which was the actual location of the Siege of the Peking Legations in 1900, and to recognize sites and street layout depicted with admirable accuracy by the sets constructed in Spain for '55 Days At Peking'. The former legation quarter is east of Tiananmen Square, bounded in the north by Changan Avenue and south by Qianmen Street. One such important site is the gate of the former British Legation in Zhengyi Road, looking remarkably as it did in photos taken in 1900. Much of the area has been occupied for many years by Chinese Government agencies. Most of the heritage buildings remaining of the old legation quarter are reconstructions after the Boxer uprising. The area's tourist potential has been little exploited. The post-Boxer former French Legation post office is the foyer of the Dongjiaominxiang Hotel. The site of the former US Legation has been renovated for up-market restaurants, bars and event venues.

Michael Simon, Moustache and O.E. Hasse were offered key roles.

Producer Samuel Bronston wanted Deborah Kerr for the role of the Baroness, whilst Charlton Heston suggested either Jeanne Moreau or Melina Mercouri for the part. However, director Nicholas Ray insisted on Ava Gardner playing her. Tom Courtenay and Peter McEnery were mentioned for supporting roles, but neither finally appeared in the film.

The cameraman on the film was originally going to be Aldo Tonti, but when he saw the sets already built at the Bronston studios, he claimed it would be impossible for him to photograph them in the way Nicholas Ray wanted. Jack Hildyard replaced him as director of photography only a short time before shooting began.

The film, which was shot in Spain, needed hundreds of Chinese extras, and the company sent scouts throughout Spain and the rest of Europe to hire as many Asiatic-looking actors as 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 film company. The company hired so many that for several months there was scarcely a Chinese restaurant to be found open in Spain and those 3 other cities.

The Japanese Colonel Shiba, played by Ichizo Itami, is the only major character among those depicted in the besieged legations who has the real name of a key participant in the actual historical events. Shiba Goro (1860-1945), then with the rank of colonel, was military attaché at the Japanese Legation. The real life Colonel Shiba emerges as a true hero and the most admired military leader within the legations in contemporary newspaper accounts and memoirs, including by English and American authors. Wikipedia has an interesting article about his career.

The production was troubled almost from the beginning. It ran into financial troubles, there were conflicts among the cast, and director Nicholas Ray argued so violently with producer Samuel Bronston that he eventually walked off the set and quit the picture, and soon afterward suffered a severe heart attack. Andrew Marton and Guy Green finished directing the picture, uncredited.

The siege of the Peking Legation Quarter was a great backdrop for dramatic effect and story-telling as the basis of this movie because it involved so many nationalities and personalities through a real crisis in an exotic location. Some survivors of the Legations, however, felt they did not have as tough a time as those caught three miles away in the concurrent more closely confined siege by Boxers and soldiers of Peking's Northern Cathedral. Defended by only 25 French and 15 Italian marines, the Catholic Bishop Pierre-Marie-Alphonse Favier (1837-1905) held out for 56 days against sniping, bombardment and mining, losing 10 of his marines killed and 500 of his Chinese congregation. There is enough material in the cathedral siege for a movie in its own right.