Victorian London was the center of high society and acts as the backdrop for Dr Jekyll and Mr Hyde. It was a city of both great wealth and great poverty, with a very large gap between the lower and upper class.

Due to this division in wealth, London was also a place of danger. At the time that Stevenson was writing his novella, London was plagued by a serial killer known as Jack the Ripper, whose crimes were widely known. By choosing London as his wetting, Stevenson adds yet another element of fear and mystery to his narrative.

The streets of London were known for being narrow and hardto navigate, acting as a maze for his characters to move within and therefore adding another atmosphere of mystery.

Fog reinforces the idea of mystery and hidden secrets
(the murder itself, but also Carew’s reasons for speaking to Hyde and being out in London at midnight).

The full moon is another gothic trope that hints at Hyde having a supernatural quality.

• Carew’s murder is described with traditionally gothic motifs to reinforce Hyde’s monstrous behaviour.
• The time of day, weather and lighting all create fear and tension.
• The fact that the murder is observed from a window increases the sense that the reader is missing key information: yet more secrecy and hidden ideas.
  

Fog reinforces the idea of mystery and hidden secrets (the murder itself, but also Carew’s reasons for speaking to Hyde and being out in London at midnight).

The full moon is another gothic trope that hints at Hyde having a supernatural quality.

The morning after the murder when Utterson starts to look for Hyde:
“It was by this time about nine in the morning, and the first fog of the season. A great chocolate-coloured pall lowered over heaven [...] Mr. Utterson beheld a marvelous number of degrees and hues of twilight; for here it would be dark like the back-end of evening; and there would be a glow of a rich, lurid brown, like the light of some strange conflagration. [Hyde’s street was] like a district in some city of a nightmare.”

“a fog rolled over the city in the small hours, the early part of the night was cloudless, and the lane, which the maid's window overlooked, was brilliantly lit by the full moon”