Thing to keep in mind?

[Ghost2012]

What are some important things to consider when building a map of a city?

[waldronate]

A city needs a reason to be there.
Everybody needs to eat and drink and have a way to get money.
Cities without sanitation do not survive.
The poor are always with us.
Rich people don't like to see poor people.
Like dissolves like.
Maps are an abstraction: a map won't show every detail, and certainly not all at the same time.
Walls are only useful against land attack.
Growth is an exponential problem.

[rpglandmaps]

building materials are localy sourced. unless some one has to money to pay to transport it, or transport networks are good (aka modern)

and few cities just apear overnight, so will have layers of time, fashion, religion and ideas.

[Falconius]

Cities develop around trade, the more trade the bigger the city. If there is no appreciable trade there won't be a city. Even towns (which are far more common) develop around trade ie as places where local producers gather their goods for other purposes.

[Azélor]

That's a vague question. You need to decide the scale , style of the map and era since transportation issues will limit the size of the city. Geography play a major role in shaping the city. Hill are used for defences, sacred places or rich people. Downwind is for the poor.

Also, you can ignore all this and move your capital in the middle of the jungle with low population, no food, no infrastructure... but with a lot of water and it could just work fine.

[QUOTE]
Growth is an exponential problem.[QUOTE]
What does that mean? Linear growth means exponential problems?

[waldronate]

[QUOTE=Azélor;405605]
[QUOTE]
Growth is an exponential problem.

What does that mean? Linear growth means exponential problems?

In my experience, people are fond of discussing a city that is "X {units}" wide/long/across. The true size of the area (and the population and infrastructure costs) that must be managed is proportional to at least the square of the "city width" metric. Simple linear associations with population can often be made (Y {thing} per Z people), but people seem to look at a map with a linear scale bar and start discussing thing as though that linear distance metric relates linearly to the problems of the city. (( !@#^& city council meetings... ))

[Wingshaw]

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.

My approach to developing cities is to 'evolve' them over time. Start with the first settlement (why, where, who) and then ask yourself what happens next in that settlement's life: eg.
-- if there are enemies nearby, they'd probably want some form of defense;
-- if they're prospering from trade, they might expand/upgrade the market or docks;
-- if religion is important in their daily lives, the church might become the focal point of social/cultural/economic/political activity;
-- if the enemies have gone away, they might reclaim the building materials used on their walls and towers;
-- if an important event took place (eg. a battle, treaty, royal celebration, heroic accomplishment, plague), perhaps they'll erect a monument or memorial
and so forth.

You should also ask yourself a whole bunch of questions along the way:
-- where do they get their food?
-- where do they get their drinking water?
-- where do people work?
-- what building materials do people use and where do they get them from?
-- what do they do with the dead?
-- what places do people visit often (eg. the market, the church, the school, the farmlands, the theatre)? This question will likely have a particularly big impact on the street layout)
-- who is in charge and from what location do they run the city?
-- how is order maintained? also, how do the authorities maintain control?*
-- where are the desirable places to live (i.e. where is land most expensive vs. least expensive)?**
-- which industries are considered undesirable/noisy/smelly/ugly? These industries may be pushed to the outskirts***
-- how do the poor and the rich interact? In Rome the rich would be patrons to the poor in exchange for political support; in Victorian times, the poor might serve the rich but be expected to be invisible

* walls and gates are not just for defense: they can be used just as much to maintain order and control over the local population as to withstand attacks from enemies. They're often also used to collect tolls, so many farmers would actually sell their produce outside the walls rather than paying the 'entrance fee', which is why medieval cities often had small markets outside their gates.

** in many historic cities, land inside the walls is scarce and valuable, land outside the walls is cheap but undesirable: economics, man This is why the centre of a medieval city is really densely built: if you're going to pay a lot for your land, then you'll want to put as much on it as possible

*** depending on the culture, lepers, immigrants, religious minorities etc. might be included in this question

Wingshaw

[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.

[J.Edward]

In an interesting twist, one major thing seems to be missing from many of these accurate statements about cities.
They seem to focus perhaps on european or US cities, in my opinion. Possibly some Chinese cities.

When you go through history about ancient cities, in so far as we accurately can, being that much of the history is myth [that many doubt] or supposition on the part of sociologists, anthropologists, and archaeologists, many cities seem to rest on the older ruins of another city. One which no one is certain why it was where it was. Though, many times there are reasons based on the ruling religion at the time of the original founding, or perhaps I would say ruling belief system. They may or may not have anything to do with the city in much of its later life. Due to that, they may bring material from ridiculously long distances away to begin the construction of some important structure. [Meaning they may, or may not, use local materials]
They may also take resources from an older city or monument to construct their newer one, as happens often enough.

As has been seen, cities may be built in what appears to be terrible locations. Sometimes this may be due to environmental changes over the long lifespan of a city, meaning it may not have been a bad spot when founded. Such that you may have what seems to be a city in a desert or in a very non-advantageous spot. Rising and lowering sea levels can lead to a city that seems to be an island near land in a way that seems impractical. Desertification is also a factor to consider.

Technological level does seem to play a significant role in what cities will be prominent in a given time period. Trains demonstrate this very well.
The advent of trains changed things dramatically as people could live further and in more varied locations, yet still commute to a larger urban center.
Travel times were greatly reduced and various options became available and changed how many businesses ran or where they were located relative to the urban center, like ice and live stock.

There are just so many factors leading to why any city looks the way it does.
I would definitely say it is always useful to have at least a basic little backstory for any city as it helps you understand something about why the city is and what its story is, the story that shaped why and where it is located.
The more accurate you want the city to be, the more information you will need for it, the surrounding area, the culture that started it, is there now, etc.

Or, you can just have fun and make some city maps.