The city is a big place. Quite a big place -- big enough that the hero doesn't have to go on a quest to be always seeing new things and meeting new folks.
There are, of course, the cities that the hero happens to happen on in the course of the quest. Which appear simple. And sometimes even are simple, but it's really hard to tell when you have limited view of a city for maybe a few days from a single POV. Minas Tirith has some views -- Pippin's, and Legolas and Gimli's brief walk through it -- but it doesn't come across as deeply complex. Unlike many an incidental fantasy city, which comes across as cardboard. Even when it's repeated. Fafhrd And the Gray Mouser's Lankhmar has a tendency to be whatever urban setting is needed for the sword-and-sorcery adventures that happens to be urban in situation. Of course there you have the situation where neither hero has much curiosity or any powerful need to know the city.
The deeply complex city tends to be home ground for all or most of a novel, or story, even if the story starts with a country character arriving there (to simplify the infodump as well as be required by the plot). It tends to be strongly flavored throughout, but this is often more a mood than anything else, and sometimes justified either in the worldbuilding or in the character POV. The city of wizards tends to be -- obviously -- loaded with wizardry. A world where champions and duels are legally valid is obviously going to have a current of violence. Of course, then you have the wretched hive of vice, the hotbed of intrigue and politics (usually with the royal court or equivalent government body attached), the city of merchants, the romantic city filled with canals and art. . . . Ankh-Morpork tends to come alive, but its major traits are heavy immigration, the home of the Unseen University, and intrigue among the guilds and nobles. Then, there is the question of what sort of city would the plot reveal.
Then, of course, there's urban fantasy. Fantasy set secretly in the contemporary world no longer hies itself off to the secret corners of the landscape where it can pass unnoticed. It puts itself front and center and uses other techniques to hide. Percy Jackson seems to have that best; the Mist expurgates the gods from mortal notice, without any input from the mortals or for that matter the gods, and so a lot of it takes place in New York City. Dresden Files in Chicago. Etc. Sometimes they come across as fantasy cities because like the imaginary ones they take on a mood.
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