Resources for a Shared Universe

I'm toying with the idea of making the city in my first post the setting for a shared-universe where friends could set stories. To that end, I've looked up a few resources on shared universes:

http://en.wikipedia.org/wiki/Shared_universe

From the thread http://www.wordiq.com/definition/Shared_universe:

Shared-universe etiquette

Originally, shared universe stories were typically written by a small group of authors (most commonly, only two) who were already on friendly terms. With online stories, material may be written by a large number of authors who are strangers to one another.

This state of affairs often gives rise to friction between authors with conflicting visions; one of the most common points of disagreement is when authors seek to promote their favorite characters over those of other authors.

To avoid these conflicts, online forums frequently develop codes of etiquette that govern interaction. These codes vary according to the forum, but often include variants on these elements:

* Restrictions on story scope, e.g. "No technology beyond that available on present-day Earth", or "No explicit sexual material".
* Statements of aim, e.g. "This setting exists to explore the interactions between characters who wouldn't usually be able to talk to one another. Please don't change it in ways that would detract from that goal."
* Each character is the property of a specific author; do not write speech, thoughts, or actions for another author's character except with their permission.
* Do not involve another author's character in certain types of subplot (often, any subplot) without that author's consent.
* Certain settings are 'common ground' and may not be significantly altered without agreement by other authors.

Threaded time

The threaded nature of some forums, and the fact that such stories are usually visible to readers before they are complete, can present a difficulty in keeping events in a linear chronology. If unchecked, story time can 'branch' in the same way as threaded conversations do. Approaches to this problem include:

* Planning out events likely to branch 'behind the scenes' before making them public (e.g. over email between authors).
* Retconning as necessary to restore linearity.
* Imposing rules on who can add to the story, when, to ensure that each author's contribution is taken into account before the next is added.
* Accepting 'threaded time', either as a reality of the shared universe or by doing one's best to ignore it.

One person even tried to turn a shared-universe writing project into a game.

Stream-of-Consciousness Fantasy City

so, inspired by this picture of the walled city of Kowloon

and recent articles I've read on architecting a fantasy RPG world, I've been thinking about what a city, a large city, in a dangerous area would look like.

Obviously there would be stout walls, and the city would, if successful, quickly run out of space. Medieval cities tended to be very crowded, as transportation was difficult. When possible, they often built up, though that was difficult at best. This city would be more advanced in terms of architecting taller buildings--you would have the forerunners of modern apartment buildings.

things would have vertical axes, not just horizontal ones. You saw this in medieval cities as well, but they rarely went over two stories. I'm thinking a medieval Taris.

Being a guardsman might be a big thing--or it might be one of those things where it's mandatory to serve a certain amount of time in the wall guard when you come of age. Or perhaps each citizen is required to give a certain amount of service.

You'd have brave (or desperate) souls who would fetch up along the outside of the wall, hoping to be protected by the wall guards, but desperately vulnerable (depending on what the threat was).

if a small community thrived there, perhaps a wall extension would be built to encompass them.

So some questions arise: what would the threat be? And what makes this city so important, this location so vital, that people would live there despite the inherent dangers and limitations?