Night time scenes were filmed by under exposing the film. Though this did leave the darkened areas darker, it still left lighted areas light, such as the sky, roof tops, rock faces, wall faces, exposed fields, etc., and those areas at night would be dark too.
During the battle sequence the exact same ground level shot of hot oil being poured on soldiers scaling a siege ladder is repeated only seconds later.
When Andrea is painting Camilla's portrait, his sleeves are long and loose. One touches the wet canvas, yet no smudge is made upon the portrait or his sleeve.
This story takes place during the time of Cesare Borgia, who died in 1507; however, the first scene of the movie--which shows Borgia with other characters--takes place in a room decorated with a fresco of Saint Michael by Federico Zuccari, who was born around 1540, and who started to work in Rome during the reign of HH Pius IV (1559-1565).
Orsini arrives in Venice and admires the city's famous sites, with St. Mark's Campanile towering over all else. The campanile seen is a 1912 restoration, but it is a faithful recreation of the earlier tower. However, the film's scene is set in 1500, and the campanile was undergoing restoration and was covered with a temporary clay-tile roof.
When speaking to Andrea, Count Verano uses the word claptrap, which originated in 1730 as a name given to the rant and rhymes as much as to say, a trap to catch a clap by way of applause from the spectators at a play.
Orson Welles, who plays Cesare, is addressed with the Italian pronunciation of his name, but when he speaks of his sister Lucrezia he pronounces her name in the English way.