There were so many sets built for this movie that some of them wound up never being used. They do sometimes feature prominently in the background, like the hairdressers, but they play no part in the story.
The book sequel was optioned, but this movie's box-office failure caused the project to be shelved. Since then, there was no movement on the future, making it as a once-in-a-lifetime adaptation, much like Mortal Engines (2018) ten years later.
Mayor Cole trips after the lights start sparking and lands on a painting of himself, smudging it. Bill Murray's character did the same thing in Ghostbusters II (1989).
The rat-like creature that chases the main characters is a giant version of the star-nosed mole (Condylura cristata), native to eastern Canada and the northeast United States. As these animals are blind, they use their nasal tentacles, known as Eimer's organ, to detect food. Although all European moles share this feature, the star-nosed mole is the only animal with such a specialized one. Apart from its size in this movie - real animals are six to nine inches (fifteen to twenty centimeters) long - nothing else was changed.
During filming, the main city was built in a paint house. It was nearly as big as a small town, and the actors, actresses, and extras were given maps.