From the Introduction
In the backstreets of the city the tenements stand side by side and often enough back to back. Only rarely is this pattern disrupted by the occasional tavern or a side ally even narrower then the other streets in these quarters. The upper stories of these buildings often enough protrude into the narrow streets even more. At times far enough that even at noon hardly a ray of sunlight finds its way to the street level.
While this among other things makes for bad living conditions it provides the less scrupulous elements of society with a different kind of walkway all together. While the shingles of the roof are often slippery and in disrepair the almost unbroken network of roof tops provides thieves with a highway out of reach for most guardsmen. And chimneys and at times their smoke provide decent cover against those few pursuers that make it to the rooftops.