Mystery is the doorway to fantasy. Dark forests, far away galaxies, roads that wind into the distance: any space that allows our imagination to play without the interference of mundane reality can be a portal. And there are few places more expectant with mystery than cities. Every road, building and doorway is a new unknown. So it's no surprise that writers of fantasy find endless inspiration in cities, and in no city more than London.

The current trend for recasting London through the prism of fantasy metaphors began, arguably, with Neil Gaiman's television series (and later novel) Neverwhere. Gaiman imagines a fantasy underworld beneath the mundane reality of London, built around the names of stops on the tube map. Blackfriars, Angel Islington and Old Bailey become characters in the underworld. It's the kind of simple, beautiful idea Gaiman has a knack for; the sort you feel you might have thought of just a moment before he told them to you.

China Miéville on the other hand has ideas you're not quite sure you get even after he's explained them. You can barely go a few pages through a Miévillian fantasy without encountering London in one form or another. Un Lun Dun is the obvious choice, but my personal favourite is Kraken, featuring truly evil underworld boss The Tattoo, his army of Fists, and a supporting cast of London police officers to go up against him. Miéville collides London's fantasy underworld with its criminal underworld, a trend at the heart of London gothic.

Ben Aaronovitch's Rivers of London is part urban fantasy, part police procedural. The book and its sequels Moon Over Soho and Whispers Underground have found an eager crossover audience of fantasy and crime readers. It's not as strange a collision of styles as it might seem; both genres deal with the traumatic events that disrupt the comfortable realities of the characters they depict. Aaronovitch's books share the cosy familiarity of many crime novels, and the fantasy elements are never allowed to disrupt the reader's belief in reality.

Paul Cornell's London Falling is a much grittier vision of a gothic, fantasy London, well balanced between its depiction of the city's criminal underworld and a horrifying fantasy reality that for most of the novel lurks just at the edges of sight. Its take on the crime genre is less The Bill, more The Sweeney. Cornell's undercover coppers and plain clothes detectives are a thoroughly seedy bunch of reprobates, and it's easy to imagine Ray Winstone taking the lead in any film adaptation.

The City's Son by Tom Pollock is a YA novel determined to bring London to life, in a very literal sense. There is a traditional coming of age adventure story at the heart of it, but Pollock's real joy seems to be in the creation of a huge cast of monstrous characters who manifest through the inanimate infrastructure of London. Reach, the King of the Cranes. Gutterglass, a servant made of trash and garbage. Pavement Priests. The Chemical Synod. Even the street lights are brought to life. Pollock's imagined London is intense and absorbing, and The City's Son is an urban fantasy in the truest sense of the word, intent on imbuing every corner of the city with life, wonder and magic.

Mystery is also the doorway to reality. Once we stray in to the forest, or reach the far galaxy, or just walk down the road, we find things both more mundane and far stranger than we had imagined. The reality of our lives in cities, even cities as full of character and life as London, is often rather mundane. We take for granted the incredible strangeness that arises from eight million lives all crowded in to a few square miles of land and architecture. At its best fantasy is more than an escape from reality, but a way of seeing again the true weirdness of the world around us. For that reason, novels of the London gothic are probably best enjoyed by people who know the city best.