Vincent Price later admitted the "wine" he got drunk with in the film was Coca Cola.
After a preview screening in November, 1939, studio heads were alarmed that the score contained nothing but period music and ordered a new score be written. Time considerations ultimately prevented this, with Frank Skinner cobbling together pieces from his score from Son of Frankenstein (1939). Only pieces of the original score survived the final cut.
The battle scenes were an ordeal to film. Principally shot on August 19, 1939 at a ranch in Tarzana, the fog machine proved ineffective in the face of high winds. The 100-degree heat caused the 300 extras to suffer and rain machines caused the soldier's cardboard helmets and shields to disintegrate. Additional battle scenes were shot on August 22 and on September 4, 1939, but the California heat continued to play havoc with the cast, crew and equipment.
According to Vincent Price, seasoned pros Boris Karloff and Basil Rathbone had decided to prank the relative newcomer. After the two evil plotters get the Prince drunk in the wine cellar, they put him into a keg of wine and put the lid on. Karloff's executioner sits on the lid until the struggles with in the barrel subside. At that point the director called, "Cut," but Karloff stayed on the barrel until the struggling resumed within as Price was starting to run out of air. After a long moment, Karloff jumped down and opened the lid, releasing a sputtering Vincent Price from his doom. Basil and Boris were greatly amused, Vincent. . .not so much. This has been documented in Mr Price's biographies and is an anecdote he would tell on public appearances.
Basil Rathbone was forced, due to scheduling conflicts, to simultaneously work on Rio (1939) during the first week of production. Rathbone's assignment on this film also effectively prevented him from being cast on the far more prestigious The Hunchback of Notre Dame (1939) at RKO.
The costumes worn by Princes Edward and Richard in their final scene are closely based on those in John Everett Millais's 1878 painting "The Princes In The Tower".