Welcome to the ecumenopolis

[Superfluous Crow]

I did actually mention farming in the Blueprints section. They have farms on the roofs/terraces. But food is still somewhat of an issue. I could go UnMet-style and just have them make some kind of semi-edible yet nutritious goo in their cellars? But seems a bit too dark for this setting. This is a thriving world, only within a somewhat limited frame.
I do actually own the Sharn book (Eberron is way awesome in many ways) and might have to take a look at it. But Sharn's towers are way more than skyscrapers can ever hope of becoming. They are practically mega-structures.
And i think Pratchett already did semi-intelligent rats before. I think rats will be animalistic but bow to the will of the Dark Piper. Could do stuff with spiders perhaps. But i think they might be a bit too small (even though they are extremely cool). But i could just enlarge them of course.  
As for taxi/traffic, maybe i should do some kind of rickshaw service? seems appropriate.
I'm pretty sure i won't have any planes because of the one world/one city aesthetic, but that doesn't mean i can't have demonic lawyers ^^

[Steerpike]

Perhaps the decaying remnants of an ancient subway or sky-rail, either mechanical or magical?  Sections could be shut-down/abandoned (instant dungeon) but others might still be operational.

[Raven Bloodmoon]

If you don't like spiders as your mystery race, you could always make normal cats secretly really wise and smart.  They're strange in their ways naturally, nearly ubiquitous anywhere there are rats, and in a city there are rats everywhere.  That would give them an instant network of contacts of vast proportions.  They are kept as pets by even the highest posts, so they've be privvy to conversations even most spies wouldn't hear.

If you don't want all cats to be sapient, you could have a special cat race that just look like cats.  Perhaps they are slightly larger or always have white paws or soemthing else distinctive.  Perhaps odler generations speak in hushed tones when they are certain no felines are near of howls at night that seem to be more than just the mating cries of toms.  Perhaps some hermit on the edge of the more civilized parts speaks of how he hears things in their catterwalling, but is thankful that it is masked in meows and just beyond perception.  Perhaps he is glad that they do continue to meow for fear of learning what they might actually be saying.

[Steerpike]

It's funny... whenever I think of cats as in tune with the supernatural/occult/hidden worlds, I think of Mieville's Un Lun Dun, where they're the only species of animal that can't enter the mystic world of Un Lun Dun - they're too busy trying to look cool.

[Superfluous Crow]

hehe, yeah, that was awesome. Actually, the roofleapers are kinda stolen from Un Lun Dun...
As far as the subway goes i do actually have the path tunnels already which don't inlcude much in the way of machinery though. Mainly just underground roads but they are meant as an allegory to subways.
Cats are a possibility. Although i'm sort of considering roaches as well, even though they are small as well. Maybe i'll make them into a race instead.
Hmm, or i could have sapient walls? :p the walls have ears you know... or a sapient street. There was some comic that did that... Nah, i should probably stay with the animals.

[Blake]

Well, if you're not going to use the idea of the stone-tasters/ sentient stonework, I'll have to adapt some of that backstory to one of my worlds.  That class is just begging to be played :P

[Superfluous Crow]

Reminds me of an as-of-yet unread fantasy series by Brian Sanderson where the mages apparently get their powers from eating small pieces of metal. You know, only with gizzard-stones instead. I'm pretty sure i'm not going to use it though.

[Superfluous Crow]

Here is something about the powers of the Cityspeakers; the mages of this setting.
 [note]Although the idea was mine it has been influenced and inspired by Kate Griffin's "a Madness of Angels" and the mini-series the Lost Room (for the Golden Key)[/note]
 
The Cityspeakers, Scions of the City
The Cityspeakers, sometimes known as Scions, have the city in their blood. Not literally, of course, but at some level they are connected to the city at a preternatural level. They become a natural part of the city, take on aspects of their home; aspects that might be considered magical by those who don't understand the true harmony of the symbiosis between man and city.

Aspect of the Faceless Masses
The city is home to many, and mos inhabitants remain anonymous aside from friends and family. Crowds is a union of disconnected individuals. When walking in a crowd, the Scion can manifest this anonymity, becoming unnoticable and practically invisible behind the cover of men and women; he becomes part of the crowd. Cityspeakers can manifest this aspect whenever he is in an area that is at least mildly crowded with non-hostile bystanders and wears somewhat mundane clothing. He doesn't become invisible as such, but he becomes absolutely anonymous; gazes do not linger on him, searching eyes disregard him, and memories do not remember him. As part of the crowd, people swerve out of his way subconsciously, allowing him to move at normal walking speed through even densely crowded areas. This movement is not directly visible to watchers.

Aspect of Sprawling Streets
Alleys and their ilk are spread out like an intricate spiderweb connecting more important streets and boulevards with their often forgotten cobblestones. No one truly realizes how many of these there are, some of them long-forgotten even by mappers, and many are known only to the Scions. The Scions can tread these paths, moving through the city with disturbing speed. Cityspeakers always know where they are, the way out and in, and where they can go; even buildings are prone to the inner map of the Scion. Their only limitations are new things, things that haven't truly become part of the city yet. These are like black holes in the  Cityspeakers mind until they have settled in.  

Aspect of the Generic Lock
Although locks require different keys, locks and keys follow the same principles and designs. Scions recognize this, and using generic keys they can fit them into any lock, changing them from their generic state to a specific state, unlocking the door, yet reducing the generic key from its superposition of all keys to a single key. Scions also recognize the similarity of doorways. Each of them create a Golden Key when they give themselves over to the city. The Golden Key unlocks any door with a typical basic lock, but instead of opening up into the room beyond, it becomes a gateway to any other similar door the Scion can mentally envision. The door closes automatically behind him, but anyone who steps through while the door remains open can use the portal as well.  

Aspect of the Ubiquitous Vermin
Although humans pride themselves on being the majority in the city, they are far outnumbered by what they call pests: rats, pigeons and their ilk. Scions recognize that these are not pests, but part of the city. They are everywhere, and their eyes see everything, and by manifesting this aspect, the Scion can borrow their vision. Pigeons can be accessed from plazas or towers, while rats can be accessed while underground. The Cityspeaker can only access these creatures when they are within a mile and have no control over them while seeing through their eyes. They can switch around all they want, though.    
   
Aspect of the Eternal Plan
The city moves with a certain rhythm and flow, and although each day is different, there are many things they have in common. The Cityspeakers are sensitive to time and place and the rhythm of the city. They are stressed in rush hours, glad when work days are over, relaxed on holidays, sad when visiting cemeteries etc. Their appetites are regular to the extreme, going like clockwork in accordance with the lunch breaks and the serving of dinner in the city.

Aspect of Unlikely Coincidence
Coincidences can be ever so unlikely, yet in the City coincidences happen with such frequency that even the unlikeliest of coincidences must happen at some point. Scions know how to be in the right place at the right time and be in this exact and very unlikely point. As such, Scions have some control over probabilities. When they need to see a friend, they will run into him in the street. If they are late for an appointment, the person they are supposed to meet might be late as well. They have luck on their side.

Aspect of Cultural Diversity
Although many diverse cultures live within the City, the Scion understands that all cultures are subdivisions of a greater culture: the culture of the City itself. As such, the Scion walks as an equal in all cultures. His traits undertake unnoticable changes, making it impossible do determine which culture he physically belongs to. Likewise, all languages of the City are understood completely, whether he has learnt the language or not. His own utterances are understood by everyone unless he takes care to speak in a specific language.

Aspect of Ghostly Emanations
Not all there exists in the city is material, and the Scion knows his way with the immaterial parts as well. He is immune to pollution as well as all other airborn poisons made by the city itself. He can sense telegraph messages in wires close by, and if he touches the line he can hear them in his head. He can also send his own messages by holding a wire, although it will become garbled if another messsage is in progress.      

EDIT: added the Aspects of Unlikely Coincidence, Cultural Diversity, and Ghostly Emanations

[Matt Larkin (author)]

Reminds me of an as-of-yet unread fantasy series by Brian Sanderson where the mages apparently get their powers from eating small pieces of metal. You know, only with gizzard-stones instead. I'm pretty sure i'm not going to use it though.

Mistborn. I loved it. Brandon, not Brian.

Does this project relate to Broken Verge, or is a completely separate one like Death Planet?

[Superfluous Crow]

It's completely separate. Mostly a place to keep all the ideas that fill up my head so it all runs smoothly :p

[Superfluous Crow]

Added two more Aspects

[O Senhor Leetz]

A few ideas -

1. A traveling circus/tent town that hops from Ward to Ward, entertaining the masses while they conduct campaigns of pickpocketing and petty theft, staying only as long as their welcome permits.

2. A capitalist/corporate part. Competing companies put advertisements and slogans every, like a peeling wall-poster that says "DRINK BOLGARTS BITTER BLACK ALE!" in a campy 1950's style with a happy family or something to contrast the rest of the world. Have "The Man" be a force of evil in the world.

3. Aspect of Smog

[Superfluous Crow]

Travelling carnival is a nice idea, and the same goes for the commercials :)
What would the Aspect of Smog entail? It is somewhat covered in Ghostly Emanations, but did you have other ideas for it?
Also thinking of adding something about lost items or something that allows the cityspeakers to have near-infinite pocket space.

[Stargate525]

How have I missed this before?

Anyway, skimmed around. You might want to look at Life After People for some inspiration. You could easily go with glass and steel; after a dozen or two years of no maintenance (ie, your setting) most of the glass is gone, and at fifty years, the blown out windows, essentially the entirety of the outer walls, have been turned into a multi-layer forest. Plenty of food if you've got one or two of these dedicated and cultured to grow nothing but harvestable crops.

Regarding things ubiquitus with cities, I an extensive subway system or an elevated rail line might be in order, devolved into underground canals or grassy strips, depending.

I'm also trying to envision how much of the city is new construction, and how much is old. When they expand, do they clear out more rooms in their building, or do they build a new building?

I'm picturing something like Feros, really, from Mass Effect. I know that's completely wrong, but...

[Superfluous Crow]

Path Tunnels are essentially subway lines, but i had the exact same idea this morning that they should be changed into underground canals.
While technological anachronisms will be present, I'm trying to avoid anything that obviously places the city in modern times. So elevated rail lines might be a bit too much.  
Having walls turned into multi-layer forests is something i have considered as well since that allows for "vertical farms" with people scaling up and down the building, filling their baskets with crops/fruit. Which is an interesting solution to the food issue.
Hmm, i reckon that if they expand they will mostly just move into new houses and modify and repair them so they can be defended and such. Of course, if houses are terribly damaged  they will tear them down or build something around them.
Also, spacious rooftops might instead be populated by multiple huts.
Thanks for commenting :)