Several hive-cities grouped closely together can merge, forming continent-spanning "hive clusters", also known as "mega-cities" outside the Imperium. In extreme cases, hive clusters/mega-cities can cover much of an entire planet's surface, forming a near-uninterrupted ecumenopolis.
A hive-city typically houses between 10 million to several billion residents, who dwell in massive self-contained habitation blocks. Designed to house a rapidly growing population efficiently, a single hab-block may house up to a million residents. The emergence of the first mile-high self-contained hab blocks in urban areas is a telltale sign of an industrialized secondary world growing to become a primary world, hive cities being characteristic to primary worlds of major economic and political importance. Overtime, as hab-blocks agglomerate into a single near-monolithic structure, they are simply built over, new and new layers of the ever-expanding city growing on top of the older strata, eventually forming a massive man-made mountain consisting of hundreds of interconnected spires and often towering well into the planetary stratosphere.
Hive cities are so massive that constructing (and demolishing) them requires a whole new take on engineering. One instrumental aspect of constructing such colossal structures is the use of macrofabrication techniques, which essentially entail the use of 3D printing on a massive scale. Demolishing or attacking a hive-city likewise requires engineering considerations normally reserved for natural formations like mountain ranges - indeed, even an average hive-city is large enough to form it's own weather system and significantly effect weather for hundreds of miles around. Simply put, the sheer bulk of hive cities allows them to weather even orbital bombardment with only superficial damage, and also means the structures are affected by natural phenomena like earthquakes more like solid mountains than cities.
Much of the material that hive cities are built up from is excavated from underneath and processed into building materials. Consequently, hive cities usually stretch at least as deep down as they are high, housing much of their actual industrial capacity and utilities deep underground, where the overlying mass of the city shields them from all but the most heavy bombardments. Overtime, some of these deep areas may flood or cave in from a lack of maintenance (since keeping track of every single hallway and tunnel in a structure of this size is extremely challenging to say the least), be contaminated by toxic leaks or otherwise rendered uninhabitable. These areas are sealed off, only the bravest and foolhardiest daring to venture in them, for all sorts of dangerous creatures and people tend to make home in the underhive.
Most of a hive-city's residents dwell within 500 levels of the planetary surface. The quarters are cramped, citizens often having to live in shared dormitories, but the Imperial authorities make sure to provide at least the basic necessities to all who reside here. Those who have distinguished themselves somewhat are assigned larger private apartments in higher levels. At the very top of the city, the Sidh society's elites live in luxurious penthouse apartments within the city's many spires.
Hive cities usually end rather crassly, having no sub-urban districts whatsoever, being surrounded by empty landscape. In many cases, this landscape is little more than a toxic wasteland choked with industrial pollution, roamed by outcasts and scavengers. In other places where the cities are carefully managed, this landscape might as well be a forest or vast corn fields, providing food for the city's residents.
Hive-worlders may live out their entire lives without ever seeing the sun of their homeworld or ever setting foot on its natural surface. Many consequently suffer greatly from agoraphobia when exposed to non-urbanized landscapes.
The massive populations of hive worlds mean extreme consumption of resources. In keeping with their frugal mindset, Sidhae especially on hive worlds let nothing go to waste, meticulously recycling everything that can possibly be recycled (including even the dead). Consequently, imported unrecycled raw materials often command premium prices - in extreme cases, a fresh piece of plastic can cost it's own weight in gold on a hive world, although such cases are exceptions rather than rule. However, any foe who might attempt to starve a hive world into submission simply by laying siege to it will learn otherwise quickly - in keeping with their autarkic, self-sufficient mindset, Sidh city-planners usually include sufficient facilities to synthesize nutrients for the population in.
The line between a primary world and a hive world is blurred somewhat, primary worlds usually having at least a couple hive-cities, but true hive-worlds usually having dozens of hives aglomerated into hive clusters. In keeping with the Sidh environmentalist principles, it's usually worlds with little to no native life that are designated to become hive worlds.
Life in a hive city
Due to the extreme population density, accomodations in a hive city are very spartan for most. An ordinary citizen will typically live in a small communal apartment with several roommates, or even a "honeycomb" - essentially a concrete box only marginally larger than a coffin, intended solely as a private sleeping space with some room to store personal articles and usually clustered in residential areas in the tens of thousands. Only distinguished citizens can claim something as elaborate as a three-room personal apartment, let alone a luxury residence in one of the hive spires.
Supplying such colossal populations with food, water and commodities is a very daunting task by any standard. For this reason, hive-worlders recycle everything zealously, even by Sidh standards. Corpses of deceased citizens are recycled like any other organic waste, rendered down to basic amino-acids and re-used in the synthesis of food or other useful commodities. In principle, each hive has enough biotech plants and water purification facilities to sustain its many residents indefinitely, but most are usually dependent on imports to at least some degree.
Life in such close proximity can be rather stressful, contributing to increased incidence of violence and crime. The vast population of the hive cities also means it is rather easy for various unsavoury characters to hide and elude justice there. Consequently, the Imperial authorities police the hives with an even greater ruthlesness than usual. That being said, the sheer number of offenses in such a tremendous population is so great that even for all their efficiency and technologies, Imperial law enforcers can only respond to and resolve an average of 10% of reported crimes. Consequently, gang warfare and vigilantism is often endemic especially in the lower reaches of the hives as citizens take law into their own hands.
Living in a hive is a rather dull life. Recreation is largely limited to getting drunk in one of the neighborhood's dives or attending the gym or sports club of the factory one works in. Although the authorities often try to organize educational and recreational off-world trips for the workers, given the sheer number of potential candidates, only a lucky few can hope to receive an invitation to such a cruise. A resident of a hive city will most probably never see the sky or the soil of his homeworld, never take a single breath of air that hasn't already passed through filters and CO2 scrubbers dozens of times and never see an animal or plant other than the ubiquitous pests that thrive in any urban environment. He will live out his entire existence inside the vast man-made maze of ferrocrete and steel that the hive city is, rarely if ever straying more than two hab blocks away from his initial place of residence.
Consequently, hive-worlders tend to have the self-contradictory trait set of adventurousness and agoraphobia. On one hand, seeing anything else than their accustomed urban concrete jungle will drive them wild with excitement and spur them on to explore, at times recklessly. On the other hand, the complete alienness of an open, natural environment will also cause extreme agoraphobia in many, discouraging exploration or even leaving their assigned quarters. While most hivers who settle on less-populous worlds eventually overcome their fears, quite a few remain impaired for much of their lives, Sidh psychiatric technicians even having a diagnosis of "hive-dweller agoraphobic syndrome". Hive cities make prime recruitment grounds especially for the Navy - hivers longing to see the galaxy and have some adventure in their dull lives strongly prefer to enlist in the Navy for the aforementioned reasons, the confines of warships providing a reasonably familiar and comfortable environment while still allowing to travel around the Imperium and beyond and seeing the many marvels to be found off-world.
While hives are generally the safest places to be in during a planetary invasion, serving as colossal and near-impenetrable fortresses capable of weathering practically any attack short of a full-scale planetary bombardment, they can likewise become deathtraps when under prolonged siege or planetary blockade. Because most hives aren't fully self-sufficient as they were originally intended to be, even a brief disruption of the import chain can lead to food and supply shortages. If an enemy can successfuly sabotage a hive-city's food production capabilities, the residents will quickly turn upon each other for the dwindling supplies and eventually be forced to choose between surrender and cannibalism. Hive worlds in proximity to enemy borders therefore usually stock enough food and water to last through a siege of several years with no surplus food produced, although the possibility of sabotaging these stores cannot be entirely ruled out.