Factual error: When the prisoners are taking a shower Roberts says to another prisoner as he goes to turn on the water "not that one, it turns on the gas" - a reference to Nazi death camps that used shower rooms to gas people. But this film is set during WW ll when this info was not known by rank and file soldiers - it only became known near or even after the end of the war.
Factual error: Rifles and more so, cannons of the type shown in the movie were not close to being invented in the 1500s.
Factual error: The US Cavalry are careful not to run into Confederate cavalry from Fort Davis, who are shown in the distance. Fort Davis was indeed occupied by the Confederates from 1861 to 1862, but was then abandoned until 1867. The film is set in 1864, so there would be no Confederate troops at Fort Davis.