When Sir Robert first arrives at Castle Sardonicus, the castle is in view in the background. The lighted windows make the castle look like a skull.

As mentioned in another comment here, Baroness Maude Sardonicus mentions Conan Doyle. Although the first Sherlock Holmes work by Doyle was not published until 1887 (written in 1886), the Baroness does not specifically mention the Sherlock Holmes novel. Conversely, she makes reference to receiving all the latest "periodicals" and mentions Conan Doyle as a writer. In fact, Doyle's first published piece, "The Mystery of Sasassa Valley", was printed in Chambers's Edinburgh Journal on September, 6 1879. He published his first academic article, "Gelsemium as a Poison" in the British Medical Journal On September 20 1879, so the movie's reference to Doyle as a writer published in journals by 1880 was accurate.

Baroness Maude Sardonicus (Audrey Dalton) talks with Sir Robert Cargrave (Ronald Lewis) about the culture in her adopted home of Gorslava and mentions they get magazines and stories from London. She mentions reading the works of "Mr. Conan Doyle", obviously referring to Arthur Conan Doyle, who wrote the Sherlock Holmes stories. This movie is set in 1880, however the first Sherlock Holmes story (A Study in Scarlet) was not published until 1886.

William Castle: [gimmick] During its initial theatrical release, attendees were given small white cards with luminous thumbs with which to vote thumbs-up or thumbs-down.

The correct term for Baron Sardonicus' condition is hysterical rictus.

Although the audience could vote on whether the main character could be pardoned or receive further punishment, technology to "choose" an ending did not exist at the time. William Castle filmed only one ending for the movie, and the "vote" was rigged by planting speakers in the theatre to vote for punishment.

The hypodermic needle and syringe were, as is said early in the movie, essentially invented by Dr (Alexander) Wood, in Edinburgh. And he was alive in 1880 (d. 1884), so could have been a friend of Sir Robert. However Wood published information on the needle and syringe in 1854, so it should have been known to doctors in London by 1880, and was by no means a "new invention" as stated a couple times in the movie.