Ok. A lot of people have covered most of the important things, but let me point out a few more.
Historically speaking, there must be a reason that the camp that became the village that turned into the city was put there/stayed there. Also, the settlement needs to have survived for a long time to grow that much.
So, let us start before history began, and the people were still nomadic hunter-gatherers.
These hunters went from place to place, hunting and gathering. They might have been looking for a good place to stay for winter, and found a spot where there were some good natural options for making some shelter (such as caves, a forest, or a good place for finding rocks), a good water-source (such as a river, stream or lake), and preferably a food-source (such as fish) that won't need them to go on long trips to refresh the store next winter.
Remember, they have their wives and children with them, so they don't want to traverse unknown, dangerous terrain. They want a nice, friendly place.
So, after finding a good place, they settle, and start a village.
It is a good place, so others want it too. That means that the first settlers (or the second, whoever thinks of it first) needs to fortify it. Meanwhile, the food-source needs to be able to feed the villagers (who are now experimenting with agriculture). If the fortifications are too hard to make, the village will probably be abandoned.
As agriculture develops, the strain on the single food source relaxes, and the village is no longer threatened by starvation.
After a while, when everyone has settled down, someone comes up with the idea of trading.
Thus begins a whole new set of challenges for our people.
They need to trade. 121
Not only for food, they probably have a lot of that, but also so that the neighbours don't look over and say "hey! they have all that stuff they don't need, let's go and take it of their hands" starting a war.
And as everyone wants the stuff they are sitting on (be that clay, fish, herbs or ornamental stuff) everyone teams up against them, so that they can trade for that ware with whoever gets it.
Also, trading brings allies. Allies mean strength and more people to do inventing, which means that you develop faster. Allies mean that your daughters can go and marry someone who is not their second cuisine for a change. This is a good thing, because intermarriage leads to all sorts of genetic diseases.
Eventually, these allied villages will decide that it is easier to take all their wares to one spot, which is close to all of them. This will become an early market, or a town centre of sorts, as people realise that if they move there they will get more custom for their wares and services.
As the small community prospers, it grows. As it grows, it creates a sort of economical vacuum where everything and everyone goes. Some people realise that they need better defences, and build some city walls. Others become more powerful, and a sort of government develops.
I think that is roughly all I can say, hope it is useful. 121