The Great Cities
The Great Cities have stood since time immemorial - imposing monoliths miles-wide, and kilometres high, titanic hives of wall and pipe. The purpose of their creation, and indeed their creators themselves, have been lost to time and the march of the ice. These Cities are home to untold millions that most never see natural light, or even light at all, nominally ruling over the entire surrounding region - a vast network of farms and mines that provide the raw resources needed to keep the Cities running, a monthly tithe from the feudal nobility of the surrounding land under the eventual threat of annihilation.
Life in the cities is not as fine as you may think. There are many luxuries and marvels built into the city itself - but those who dwell within it often have little understanding beyond the minimum to make them work. There is no such thing as fresh food, with all food being preserved meats and dried vegetables that are added to the city stores for its hungry populace to chew through, and any break in supply chains means that people are going hungry. The cities are also incredibly high density in most regions, packing people together like sardines with little in the way of space, privacy, or personal expression - since there is just as little to remove heat or smell, the culture has universally adopted a strict hygeine routine, and usually favours minimal coverage clothing both to deal with the heat and as a result of their limited to non-existant taboos surrounding each other's bodies. In fact, living on the outermost regions of the city is considered the domain of the higher classes, as there is access to sunlight, fresh air, and even natural weather. Such upper classes often favour fuller clothing, and have more social mores - they also tend to suffer the morose of their environment far less than the lower classes, being able to see more than simply miles of twisting corrdior or cyclopean gaps formed between spaces of not-quite-rock.
The primary forms of employment within the Cities are split between labourers that produce the clothing, arcane weaponry, and other tools in use by the City; quartermasters and conveyors of the resources that must be ferried between segments of the City; those of the machine-cults dedicated to maintaining the bizarre workings of the City using knowledge passed down between the generations; and researchers that attempt to study and understand all around them. While the cities maintain fighting forces, these are both rare and elite, being outfitted with the finest technology of the cities. Since this is impossible to create or maintain without access to a City's forges, these super-soldiers are required to dedicate their entire life to fighting and service, as they will most certainly be priority targets to their foes and each death represents a blow to the City that sponsors them. They are almost only fielded alongside small forces of riflemen, and are expected to receive the support of nearby Lords in open conflict. That being said, they are far more present as a simple threat: any Lord that refuses the demands of their City is owed a visit by a single or even group of these soldiers, and there is little their house guard can do to stop them.
To say those within the cities have a complex relationship with Mutation would be a lie in the extreme - it is simply bad. If a person is discovered to have a mutation, they are exiled from the city, or even killed. If they are too important for this, or are expected to encounter mutation at some point (as is the case for nobility or soldiers) then they will have the mutation surgically removed, and their loyalties forever questioned from then on. For this reason, any suffering mutation will typically do their utmost to hide it from their fellows, often only inviting further suspicion in the process.
To those outside the Great Cities, such places often might as well be myth. Few live so close or otherwise intertwined with them to see evidence of their presence, with most either including them in their religions or believing that that is where they reside altogether. They are looked upon with much the same gaze that a member of the Christian world may view heaven, including those that are told of it and view its description with nothing but scorn.