Like many of the Imperium’s great hive cities, Desoleum Primus is formed into a mountain of plasteel resembling a termite mound of impossible proportions. The highest point, known as the Apex, is host to the seat of power of the hive’s, and indeed the world’s, ruler, Lady Aud Killian, the hereditary Lady Desoleum. Most of the actual ruling is conducted through the Consortium, a body concerned with profits and the continuation of business concerns that have stood for countless generations. Though there is much infighting between factions within the Consortium, there is relatively little actual war between the members; most factions instead use proxy forces such as gangs or bribed Sanctionaries when actual violence is required.
The uppermost point of the Apex penetrates the edge of the atmosphere itself, with a spike of indestructible metal called the Spine reaching several kilometres into the void. The Spine descends through the middle of the hive into the depths underground, penetrating below even the Underhive and the Dark with no termination that any have found. Many theorise that it is vital to supporting the hive beyond mere structural support; legend even has it that "the Spine always existed, and it is the hive that grew around it". The Apex houses the cream of the hive’s nobility along with powerful Imperial representatives, and an exquisite Ministorum Shrine that serves them as well as sufficiently faithful offworld dignitaries. There are several small shuttle pads along the outer skin for the elite, so they do not have to pass down through the squalor and dangers when travelling away from the hive.
Beneath the Apex is the Upper Hive, sometimes called the Pinions, where the great houses of the Consortium operate and the majority of the hive gentry live. The worker-serfs living further down-hive can only dream of the luxury even the lower tiers here exist in, the most privileged enjoying actual sunlight filtered through windows affording them grand vistas of the expansive cloudscapes below. Despite the unimaginable luxury of these areas, they are atrophied and declining. The ancient, little-understood Archaeotech systems that sustain them are slowly falling to disrepair. Growing areas of the upper reaches are now sealed off from habitation, though some still find use for secretive meetings or illicit activities. Like their environs, the oath-cogs of those within the Upper Hive are incredibly ornate as befits their high station, but many are becoming tarnished and even inoperable.
The great bulk of Desoleum Primus is the Main Hive, often referred to as Desoleum City, an area that accounts for the large majority of its population and industry. Businesses operated by the houses of the Consortium dominate this region, and these oversee almost all facets of life in the hive. Those areas not set aside for hab housing contain enormous factories producing the hive’s main exports. Massive refrigeration systems cool the upper levels of this area, for the factory heat is intense. Many metres of greenish frost often accumulate along the ceilings, making for a thick barrier between the upper and main hive. Huge icicles of frozen, polluted water also form here along the hive’s outer edges. These often break off after forming into chunks hundreds of metres long, creating terrible scars in the hive’s outer skin as they rip down its length with screaming sounds audible even in the hive interior.
This area is host to all classes, and is the highest level that Consortium-affiliated Gangs can expect to travel unchallenged, so long as they are about the business of their masters and not intent on overt violence. The line between the Main Hive and the endless reaches below is indistinct and in places highly variable.
The lower one goes, the more the air becomes more stale and humid, and the light more dim and unreliable. Rain comes from foetid waters dripping down through the levels, coating all with oily liquids. While there is no firm divisor, there are plenty of guards to prevent "Underhive scum" from roaming too far above their station; some areas even have huge barricades and fortresses to protect valued areas situated in the lower Main Hive. Along the rough border areas are thriving protection rackets that keep the scum away from the trading posts and habs. The border is defined more by the extent to which the Sanctionaries are willing and able to patrol, the area beyond becoming the domains of gangs, Outcasts, and worse. Their main concern is that the scum does not drift up to "invade" the Main Hive and they have no concerns about who passes downhive or why.
The true depths begin at roughly ground level, where a few ill-maintained roads and passages exist leading to the surrounding wastes. The Underhive is home to those who have abandoned their oaths as well as those who know nothing of such things. The gangs here are the worst to be found in the entire hive; names such as the Oath-Breakers and the Hive Worms are used to scare up-hive children to bed and Main Hive merchants to pay up. Most hivers will never hear of such hells as the Cold Reaches or the Red Drippings, let alone visit such vile places, but many have heard of the Cascades or the infamous Gorges, where the inhabitants are said to be ravenous for anything or anyone they can capture.
Below the Underhive is an area that makes that region appear organised and law abiding in comparison. This is the Dark, extending so far into the planet’s crust it is impossible to chart, though the Spine is always present no matter how far down one ventures. Huge lakes, created from aeons of effluent draining down from uphive, form in many regions the farther down one descends. Monstrous creatures dwell in these seemingly endless depths, hidden in the black waters until revealed by the ghostly lights flickering on their massive hides when they rise to feed.
Hivequakes periodically strike Desoleum as its massive bulk shifts and settles, making the Dark even more dangerous; caverns collapse as supports fail, but ancient areas are revealed anew, drawing many to seek out any treasures within. Legends say that "wondrous archaeotech devices exist in the Dark, such as grav-repellers that can lift starships, inexhaustible power sources that draw from the very tension of reality, and nightmarish guardians that kill with whispers. To find so much as a shard of such wonders would elevate even a foul stumper to the highest echelons of the hive’s society overnight". There are large settlements here, larger than any above would imagine, where mutants, heretical worshippers, and other terrors live and scheme and fight with each other. A few dare venture into the Dark in search of treasure, thrills, or the monsters that roam in the sputtering light and heavy air. Fewer still return.