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.

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.

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.

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.

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.