Ian Rankin leads the way down a dark, stone stairway into the bowels of his city. He takes a turn, passes under the massive stone arch of a bridge and, after walking another 50 yards, turns and stops.

“There it is,” he shouts above the traffic. “Anyone who dies in Edinburgh starts their death here.” We have arrived at an anonymous 1960s brick building, its two stories dwarfed by the towering structures around it. Inside, piled up against the highest window, is a stack of pots that look a lot like paint tins through the opaque glass.

But it’s doubtful that they ever contained anything quite so benign as paint, because this is the City Mortuary. Embalming fluid, perhaps?

The author is in his element. This is precisely the kind of Edinburgh that Rankin loves. “You go in there,” he says, pointing at a metal roller-shutter, with the enthusiasm of a genuine aficionado.

“This is the main dock. When that’s opened up all you see are fridges where they slide the bodies in. And there’s a separate room for the Decomposing Room, where they keep the really bad ones, which have been sitting for a while.”

This is Rankin’s world. We’ve been walking for an hour on the trail of Inspector John Rebus, the author’s most famous creation, and here we are, on the Cowgate at the centre of the Old Town.

From the first of 17 novels, played out in the tunnels underneath the public library, to the last, a body at the foot of the castle rock, most have centred on two or three square miles of the city centre.

Like Robert Louis Stevenson, Rankin sees a precipitous Edinburgh around him and imagines this location as the deepest point in a canyon, its high walls built and rebuilt for hundreds of years since the Middle Ages. Even in daylight, it is a dark place and, if Rebus is to be believed, the narrow road running along the bottom is nothing less than a river of blood.

Here is the great detective in Dead Souls, standing on South Bridge, which rises high above our heads. “He stopped at some railings so he could look down . . . There were clubs still open . . . teenagers spilling on to the road. The police had names for the Cowgate when it got like this: Little Saigon; the blood bank; Hell on Earth.

Even the patrol cars went in twos. Whoops and yells: a couple of girls in short dresses. One lad was down on his knees in the road, begging to be noticed.”

It is all pretty much true if you come to this spot at midnight, though it is quieter for a stroll on a summer’s day. If you want to linger awhile, you can take an imaginary drink with Rebus and Dr John Curt, his pathologist friend, in Bannerman’s Bar, where “Rebus would swear he could smell soap and surgical alcohol wafting up from Curt’s hands”.

It is these collisions of fact and fiction that bring alive the dark atmosphere of the city. Not far from this spot, up and over the Royal Mile, you could visit Fleshmarket Close (site of two corpses, in the novel of the same name) or arrange a tour of Mary King’s Close, a bricked-up medieval ginnel (the hang-out of a brutally tortured body in Mortal Causes).