Think of a tree stump. Picture the irregular pattern of rings that run through the wood, each ring telling you something different about the life of this tree.
Now think of an old city from above. Follow the tracks of the roads and walls, and you will see a rough mirror image of the tree rings on the stump.
It might look a little like this:
Depicted: One million opportunities, many of them fatal.
Layer per layer, century after century, wall by wall, a city grows. Except that Ankh-Morpork wasn’t built quite in that way. As with many things on the Discworld, chaos was a much better alternative to order. Thus, when Terry Pratchett and Stephen Briggs ventured to create a complete city map, they first had to figure out which streets were in which novel. Ankh-Morpork had to be stitched together from thousands of pages, but the result was something akin to a real map:
In The Colour of Magic, Rincewind could run where he liked, because the city was still only vaguely mapped in my head; by Night Watch, the movements of Sam Vimes were plotted and timed on the Mapp. Indeed, the rooftop chase across Unseen University at the start of that book was written with reference to the Mapp and the limited-edition model made by Bernhard Pearson. The route and the lines of sight had to be right, otherwise people would complain.
(Source: The Art of Discworld)
The Discworld had just become a little more real. Not too real, because that would kill the magic, yet real enough for readers to follow their favourite characters through the streets of Ankh-Morpork. And perhaps meet some of the less favourable ones.
It could be a lot worse. You could be in the Shades. Or at least your body.
And still Ankh-Morpork and the Discworld are getting more real. They bend reality their way so that it won’t do any damage (or allow the horrors from the Dungeon Dimensions to break through once more). Every new novel adds another layer to the Discworld. Characters and technology develop, unknown alleys and routes appear… and so the map will never be complete. Thankfully.