Goofs

The hilt of Bilius's sword is white. However, when Horsa steals it from his scabbard, the hilt is suddenly black, and remains so for the rest of the scene.

There was no letter "W" in the Latin alphabet.

Brutus warns the senators that if Caesar is allowed to go on as he has, "the Roman Empire's finished". The Roman Empire hadn't started yet. This is set at the end of the Roman Republic.

Rome did not have eunuchs as an institution that way (e.g.) Byzantium did.

Early in the movie when the Romans are attacking the Briton's village, the Romans are wearing red capes hung from the shoulders. Usually worn by officers, the capes wouldn't be worn during a battle as they would hinder the soldiers as well as being used against them by the enemy by being pulled back, over their heads etc.

At their first appearance, Caeser's legion marches while calling out a comedic left-right cadence of "sinister-dexter-sinister-dexter." Sinister and dexter refer respectively to the Latin terms for left and right, but they march with the handedness incorrectly reversed: calling "sinister" on each right footstep and "dexter" on each left footstep.

Towards the end of the film, the British slaves are escaping out of the window. As they rush over to the window, one man's costume flies up, and you can see the modern red underwear he has got on.

When Cleopatra rolls out of the carpet and causes the table to collapse, the table falls without her touching it. Clearly it was pulled by strings.

When Cleopatra gets out of her bath, you can briefly see her bikini bottom.

Near the beginning, Hengist and Horsa are watching a young cave couple dragging each other around. Hengist is pouring Horsa a drink and you can see him pour, raise the jug, then Horsa takes a sip from his cup. All this time it is obvious that no liquids are in the vessels.

Ancient Romans did not wear underpants, and certainly not red underpants.

When Caesar and Calpurnia are in bed together, the bed has pillows. The Romans didn't have pillows, they had a curved headrest made of stone or metal. The beds were also basic, not the lavish mattress shown.

Early in the movie when the Romans are attacking, Horsa makes a comment about obtaining help from Boudica and her army. The movie is set during the life of Julius Caesar who died in 44BC while Boudica died in 60AD so Boudica could not have been around during the time frame of the movie.

Mirrors in Rome of Caesar's time were polished metal, not (breakable) silvered glass.

There was no "emperor's throne" in Julius Caesar's time, nor was he an emperor.

Eucalyptus was found only in Australia and New Guinea until the Europeans arrived there in the 17th century.

Seneca (Charles Hawtrey) is wearing NHS (National Health Service) issue spectacles which were not introduced until 1948.

When Hengist (Kenneth Connor) pours the ashes over Julius (Kenneth Williams).. you can hear somebody laughing in the background.

@ around 55 minutes when Caesar and Antony are facing the camera discussing Cleopatra the shadow of a boom mic can be seen passing in and out of the shot (just to the side of Mark Antony's right arm).

Mark Anthony says after the first failed assassination attempt that Hengist "fought off the top five gladiators in Rome." However, the attack was carried out by his bodyguard, as Mark Anthony well knew, the elite Praetorian Guard, not gladiators, who were criminals and ex-slaves forced to fight in the arena.