Hive City

Hive cities are massive structures, miles high as they are miles wide at their bases. Found on many human worlds, especially on the Hive Worlds of the Imperium, they are self contained cities yet from the outside look nothing like one. Slums are built up around the exteriors of them, and long forgotten ruins lie below them, many utilise geothermal energy at their core. Each have been built up over millennia on the detritus of previous buildings sitting under the city. 
Many hive cities are packed with people, businesses, military bases & naval ports, along with a plethora of factories and life support mechanisms as big as a cathedral. Many shrines, great & small, are dotted around. Each is fed by corpse-starch, and other such, factories, with exotic foodstuffs & water brought in from other worlds (usually Agriworlds). Sometimes gasses are imported too, both for business & for breathable air.
Life exists here, usually excluding mutated creatures boiling up from what is known as the Sump or Undercity, and is both cheap for the masses. So much life exists in each usually numbering between the billions to trillions in a single Hive city.

Demographics

The majority of the population are workers who are assigned tasks by taskmasters no better than slave owners. A minority of other groups (mentioned later) keep the hive city in check, at least on paper. A rare few belong to powerful castes with influence in the Imperium, and a few off-worlders who are assigned to the city as specialists.

Government

The hive city is run by a noble class that hold charters to run the city & it's inhabitants in the name of the Emperor. As long as any tithes are made, then they can run it as they see fit. There is a planetary governor in one hive city who usually has complete control, or a council in rare cases. In many ways it fits the idea of a feudal society.

Defences

The defences of a Hive City is surprisingly effective, even though many have fallen over the millennia.
The primary defences are found to be the sheer size of the city, and also it's biggest weakness. Since it can't make sufficient food or water during a siege it can disable a Hive City pretty quickly. What makes it so hard, is the point defences built into it. These include laser & flak cannons, and other weapons found nowhere else, save on voidships & military bastions, jutting out at every level.
The second defensive aspect is the sheer amount of manpower available. Whole Imperial Guard & Planetary Defence Forces (PDF) are found stationed in them, along with Sisters of Battle, Adeptus Arbites, and even recruiting grounds for Adeptus Astartes in a few instances. This is all backed up by a regular Adeptus Mechanicus cadre here & there, a few naval stopovers once in a while, and the crime gangs that can be found at the lower levels. These are backed up by the nobility and their personal guards, along with psykers of the Adeptus Astra Telepathica.
A third defence most overlook is the surrounding regions of each Hive City. The world itself is usually highly scarred in some way, usually by unrestrained industrial practises. This will have scarred it in a way to make highly hostile environments filled with highly mutated lifeforms, chemicals and radioactive materials dumped in vast quantities, and weather patterns that are highly volatile.
A note that has to be made is that some hive cities have one advantage only the Forge Worlds can match or exceed is the capacity to produce war materials like no other. Indeed, during the Second War of Armageddon, and now the ongoing Third War, local guard units were bolstered by Chimera APCs & Leman Russ battle tanks fresh of assembly lines without even primer paint on them.

Industry & Trade

Each hive city is a major exporter of goods throughout the Imperium. Everything from buttons to warmachines of some kind are produced by one hive city or another, and all have the facility (to a greater or lesser extent) to produce corpse-starch in vast quantities. All of these find their way through both the system they are produced them, to the furthest reaches of the Imperium.
Because of their nature each hive city has to import raw materials for their industry, along with some foodstuffs to supplement the people, and fresh water too. With this, they trade these goods with other star systems, planets in their own, or even with other cities on their own world.

Infrastructure

Massive factoriums produce goods for the Imperium at large and are scattered at the lower tiers away from the powerful. They export countless products for use, while receiving in needed supplies, along with the odd luxurious item for use by the powerful. Some hive cities produce one type off goods, while others produce multiple different goods depending on it's original design.
Many supply troops for forces of the Imperium. As many have gang problems these make good soldiers who know which way to hold, and fire, a gun correctly, while think of nothing of killing others on the spot. Sometimes Adeptus Astartes choose from a city for the same reason, and other groups such as the Adeptus Sororitas & Adeptus Arbites choose recruit for the same reason.

Districts

At the top (and the cleanest region) is the noble houses, and a planetary governor. They rule a specific hive with an iron fist, and each hive will have a specific family, or small set of families in each.
Below them is equivalent to the minor houses, and high ranking Imperial citizens such as the Astropathic Choir, high ranking techpriests, military / naval officers, Adeptus Arbites Lord Marshal, and other such groupings that try to keep everything moving as they should.
On the next level is lower ranking individuals & groups, with more of the finer sides of life available, and less dirty & dangerous jobs. These take up many square miles of the levels here, and make up their own small districts, though no two hive city is identical in layout here or even below.
The last main region is the lowest of the low, at least officially. These are the standard working areas, and the average Imperial citizen will have lodgings here. Businesses of all types can be found, and massive factoriums are here, producing everything from laser lenses, to medicine, to corpse-starch, in abundance. These never stop in their production.
Unofficially there is one last level, the Sump or the Underhive. This is usually the remnants of the original city, built all those millennia ago, and is highly dangerous. It is filled with effluence pools made up of nuclear, biological & chemical runoff from the levels above, and the pressure exerted by the higher levels has broken massive sections of it by the shear weight, and the weight of time. Much archeotech can be found in these depths, along with highly dangerous & mutated creatures including abhorrent human mutants. Very few ever come down here, yet a rare few do. The prices fort discovering strange materials formed by the waste, along with time & pressure can alter them into useful chemicals. Also, the archeotech that can be found is from ages past are worth their weight 10+ time over.
The slums on the outside are made of those remnants that have been forced to remain outside, forced out as punishment from the hive itself, or general outcasts and/or on the run.

Assets

The greatest asset found in any hive city is the population. With literal millions at least per hive city, manpower is no problem. There is always someone to exploit and force into gangs, whether to work or for crime, and for military purposes.
The second most important asset is the factoriums found in the lower levels. These wretched places are nothing short of sweatshops, with generations carry out work by rote. Each is slightly better than slaves that work in them, but they can churn out items at a scale never seen before in human history. So much so, many hive cities have close ties to the Adeptus Mechanicus who supply the equipment, training & blueprints for items to be made of one sort or another. This frees up Forge Worlds to concentrate on other areas of production, and even experimentation.

Guilds and Factions

Many guilds are found throughout each hive city depending on it's nature, and a few factions are ubiquitous. Manufacturing guilds can be found with ties to groups like the Adeptus Mechanicus who always has a presence somewhere in the city.
Besides the Adeptus Mechanicus, there are a number of other organisations too:

• Adeptus Arbites
• The Ecclesiarchy
• Adeptus Astra Telepathica
• Adeptus Sororitas
• Adeptus Administratum

Sometimes Naval assets are present, as well as the odd Adeptus Astartes, and Chartist Captain groups & Navigator families.
One faction lies below all others and can be a threat if they get to big. These are the criminal gangs that operate in the lower levels, and can easily cause untold damage to infrastructures within the city.

History

Every hive city has a different history, though many have a similar beginning. When the Hive World's ecosystem starts failing the cities become safe havens for society, at least at the beginning. Over a period of centuries the world becomes too hostile for human life, at least those not mutated enough to survive outside for extended periods. When this happens, the inhabitants of the new hive city starts to build upwards instead of outwards, and the city keeps growing taller and taller and taller, until the upper spires are high above the atmosphere. The older lower levels by this stage starts to fall into decay, and those that can be maintained somewhat eventually become used by those with little to gain in life.

Architecture

Hive cities are a mishmash of different styles, usually a conglomeration of different centuries or millennia of technology & design. In many cases these are boosted by retrofits when technology catches up, or needs jury-rigged. Eventually all follow a gothic look to the design built over elements of minor ethnic details hidden or crumbling away over time.

Natural Resources

There are no natural resources left in the hive city, with the human population being the only real resource left.

"The Arbites came through recently. Dragged of half the workers there. Its said that they were mutants, belonging to some cult meeting in the Sump, and they weren't 'pure'. Said that the ones that were taken away had Xenos blood with strange marks on their bodies, and that something got to them.
Strange though, they were some of the best workers there, and the overseers barely had to whip any of them if ever.  Though saying that, there was some heavily armoured woman present beforehand had conducted some kind of tests on them before the Arbites arrived. Se had what looked like a writ of some kind that most were terrified about. Think it was the Inquisition or something.
Also seemed to be related to the stories of the Sump having some sort of creature there, seen in the shadows with four arms that could cut through armaplas with ease. Was supposedly seen eating initially from the vats used in the corpse-starch mincers, and had escaped when the overseers came through with a techpriest. Makes you think what's down there below us. Is it some type of mutant creature? A native to the planet or the Sump? Maybe an escaped experiment? We'll never know in this warren of a factorium. All we do is supply those cans of corpse-starch to the Imperial Medicae out near the system edge. Better not to think or the Arbites & this woman might appear again. Don't want that do we?"
Unnamed worker, that was found torn apart by his co-workers and overseer, three Terran hours later at the edge of the storage facility he worked at.

Type

Metropolis

Owner/Ruler

Emperor of Mankind

Ruling/Owning Rank

High Lord

Owning Organization

The Imperium of Man