I was trying to think of other "Big" stuff in fantasy fiction, but I don't read a lot of fantasy fiction so the only one I can think of is in Ash: A Secret History - you've got the sun being blacked out across most of North Africa and Europe. That's pretty big.

My homebrew campaign world had a cataclysm in which one of the greatest cities of the previous age disappeared into a canyon (think Grand Canyon but longer, wider, effectively infininately deep) that split the continent in half, and was so deep that it connected with the Plane of the Abyss, (which was effectively the negative elemental plane, combined with a demonic plane in my campaign) and Plane of Fire*. In one of the early adventures the players set through a portal that use to connect to that city and discover it full of undead and demons, and quickly leg it back through the portal.

There is a bridge several miles long and a couple wide, that spans the Abyss, held aloft by magic and guarded from the creatures that rise from the Abyss, and a city/shantytown that runs the length of the bridge, it obviously being a major trade nexus.

That's pretty big.

*Comsmology was that the planes were layered on top of each other so the Elemental Plane of Fire was at the centre of the Planet. Elemental plane of water was in the deep ocean (constantly creating water that was then consumed by the Abyss at either end, poored down until it hit the plane of fire, which turned it to a mist that rises from the Abyss to the plane of air), Earth plane started a few miles below the surface, and Air in the upper atmosphere. You could actually travel to any of them via conventional means if you were suicidal enough. The Abyss was between the Earth and Fire planes.

Interesting idea.
 

Quote from: Eric E.Tectonics. In reality, most planets have those wonderful tectonic plates causing things such as mountains and earthquakes and tidal waves. Tie that into volcanoes, and I'm just not sure if I would include these real-world things.

Leave 'em out.  Consider it part of the fantasy element.
 

QuoteWars. Wars are fought through the city. There are areas between different regions where the city has been modified so major wars can occur. There the streets run red with blood, the cobblestones lined in blood. Massed units of troops are only effective in broad thoroughfares (unsure of what to call them yet, Bloodstreets possibly). However, smaller unit city wars also occur, where homes and shops become battlezones at the worst of times for the inhabitants.

Nah.  Wars are not fought in agreed upon zones.  Ultimately, a war is just a name for using force to get someone to do something you want them to do.  That doesn't always happen in an arena setting (which is, essentially, what you've got here).  That is, except for cultures with a very, very strict code of honor.
 
A global city would have two impacts on war.  First, it'd be bloody, messy, and ugly.  Any engagement en masse would include levelling your opponent's cover.  That means lots and lots and lots of collateral damage.  Lots.  Break buildings and dead civilians everywhere.  Rumors of war would be grim, indeed.
 
The second would be to devalue hordes in favor of skill.  Navy Seals and adventuring parties would rule the day.  This is a really good thing for a D&D setting.  There's always a castle to raid, buried sections of the city to explore, etc.  The world is a dungeon.  Sure, adventurers may not be well liked, but they'd make a whole lot of sense and be necessary.

Yeah, I don't think I would include the real world tectonics.  I don't think that would be an issue for many folks anyway.  Thinking about it, it seems like many fantasy RPGs do this same thing (consider all of the underground civilizations, it would take immense magic to stop tectonics to completely protect cities and passages).

Good point on wars.  Small units (parties of adventurers as hired specialists) would probably be a preferred way for many of the rulers.  After all, who wants to completely destroy the area you are taking over?  Also, this gives a great reason for adventuring parties, rather than hordes of warriors.  I may have to review the Black Company Campaign Setting for some ideas on tactics and such as well.

Quote from: Eric E.In the back of my mind, I keep working on a fantasy setting where the whole world is covered by one city.  All of the different climates and environments run through the city.  It isn't the same style through the whole city, parts may be open like large agricultural fields, but the world is covered in the city.  Some areas of the city have fallen or sunk, and they were built over.  So you have catacombs that may or may not be inhabited.  The city has been around since recorded history.

Scope this setting out... yes I know it is MtG, but Ravnicca was all about a vast world spanning city...
Ravnicca
and scope out these pages for some pics that may help inspire a few ideas...
Life in the Big City and The Essence of Land.