Thing to keep in mind?

[Falconius]

Falconius is right, trade is a key reason for cities to exist, but it isn't the only reason. Military outposts can become cities (Manchester, UK), religious enclaves can become cities (Münster, Germany), refuges can become cities (Venice, Italy), prison colonies can become cities (Sydney, Australia), resource rich camps can become cities (Dawson City, Canada), and so forth. While trade definitely had a role in all of these, you can see that there can be a somewhat tangled story behind a city's origin. There may be one reason a settlement begins, but there can be many reasons for it to grow into a city.

Wingshaw

Indeed, I was also thinking of exceptions for places that are simply mandated to be cities (or otherwise important) for whatever reason like the Forbidden City, or Kyoto, or even Jerusalem. [Edit: How could I miss the most obvious example in Washington DC?! !] But those also become trade points, and I didn't feel like including a bunch of caveats. Nevertheless whatever reason a location is founded is not the reason it becomes city big or sustains itself, that's all down to trade.