The filming location is a valley near "Camerata Nuova", a small Italian town, close the border of Lazio and Abruzzo regions, and Almería, a small town in the South-East of Spain where a lot of movies, including spaghetti westerns, were filmed.
Fourth western starring Terence Hill and Bud Spencer. The earlier oaters they starred in were "God Forgives... I Don't!" aka "Blood River" [God Forgives... I Don't! (1967)], "Ace High" [Ace High (1968)], and "Boot Hill" [Boot Hill (1969)]. Apart from small parts in the Italian sword-and-sandle epic "Hannibal" [Hannibal (1959)] billed under their birth names, at the time of this movie, Hill and Spencer had actually never co-starred together in a movie which wasn't a western.
The iconic scene where Trinity (Terence Hill) slaps a bad guy while quick-drawing his gun has been only slightly accelerated; the actor had to actually practice those moves for a week, at his house, on a pillar of his veranda. Later he admitted that his wife was actually worried in seeing her husband slapping and pointing a gun for hours to a piece of concrete.
The wooden bed-like rack that Trinity (Terence Hill) lazily lies on and is dragged around on by his horse is known as a "travois". (Bambino at one point calls it a "litter".) Wikipedia defines "travois" as "a frame for restraining horses...a historical frame structure that was used by indigenous peoples, notably the Plains Indians of North America, to drag loads over land". Photos of Hill lying on the travois were one of the significant images associated with the picture and were seen in production stills and many movie posters for the film.
Trinity's (Terence Hill) nickname was "The Right Hand of the Devil". (The type of gun Trinity had was a Colt 45.) Bambino's (Bud Spencer) nickname was "The Left Hand of the Devil." (The name Bambino is Italian for "baby".) Both men were expert shots. "The Devil" was a nickname for their mother, a New Orleans madam.