All scenes shot in the marketplace/village were shot at Bray Studios on the revamped set that was constructed for Horror of Dracula. Once this film ("Stranglers") was completed in 1959, the set was completely torn down. The sequences where 'Guy Rolfe''s character goes to meet Allan Cuthbertson's character, the capture of two of the thuggees in the process, and the caravan sequences were shot at the sand and gravel quarry in Gerrard's Cross, Buckinghamshire (England). The tiger hunt and the scene where Rolfe's character finds more grave sites was filmed at the Callow Hill Sandpits in Virginia Water, Egham in Surrey.
Guy Rolfe's Captain Henry Lewis is partially based on William Henry Sleeman, who declared war on the Thugees in 1835 and with the hanging of thousands of its leaders and members was able to eradicate it by the 1870s.
The fight between the snake and the mongoose wasn't in the original script.
The military men of the story are not part of the British army but the East India Company, which virtually ruled India for two centuries. With roots in the 16th Century, the company was granted a monopoly from 1708 to 1857, when it became the largest global trader in the world, dealing with cotton, silk, indigo, and even opium.
The script was written in two weeks.
The Thugee cult can be traced back to the 17th Century with roots going as far back as the 13th. According to Wikipedia it is estimated that they murdered two million of their countrymen. According to the film's end credits, one thug allegedly claimed 700 victims.