|Pieter Bruegel's Tower of Babel|
The ruins of a super-tall arcology stand here. From the base of the building to the 100th floor (1500ft), the exterior is surprisingly intact. Above the 100th floor, less and less of the building remains until only a small portion exists at the 300th floor (4500ft), creating a knife edge or spike appearance. There is no indication of how tall the arcology originally was; however, for aesthetic’s sake, the building was 1.3 miles (7566ft) high, consisting of 504 floors. There are also 50 sub-floors. The arcology sits on a square base of 1150x1150ft (aprox 15% of the original height). Obviously, the building is visible from quite a distance.
While the super-strong exterior has remained mostly intact, at least around the base, the interior has fared much worse. The building still runs on functioning geo-thermal generators which gives access to partially functioning elevators, minimal interior lighting, and other energy needs. However, there is no full access to any of the floors; only portions remain unblocked by debris or fallen floors. From a play perspective, this is so that you don't have to make a 1150x1150 square map of 100+ floors. All in all, I'd say there are a maximum of 20 accessible floors; these floors can be of varying sizes to increase or decrease the amount of exploring required. Travel between floors can be achieved by unobstructed stairs, functioning elevators, or makeshift ropes and ladders. There are also no computers. All functions, even mechanically and electrically driven functions, appear to be manually operated.
Each floor is 15ft tall, made up of 13ft of open living space and 2ft of engineering space (plumbing, lighting, electrical, etc). The interior of the building is made up of living spaces around the entire perimeter of each floor; this allows all living spaces to have windows to the outside. The interior is made up of manufacturing, shops, restaurants, offices, and recreation rooms. Living spaces are generous at 50x50ft (3025 square feet), with customizable interiors. Retail and business areas are of varying sizes, however, there is generous open space (perhaps 100ft, more or less) separating the business section from the residential spaces, filled with wide walkways, small trees, bushes, and grass. The first floor contains a open park space that occupies the entire interior of the building as well as 4 floors (60ft) of vertical space. It is filled with open park space, tall trees, and simulated breezes. Residential spaces surrounding this open space that are not on the "ground" level open onto 20ft communal balconies that overlook the open space. This open space occurs every 20 floors after (ie, floor 1, floor 21, floor 41, etc.). Originally, the arcology housed 24 of these parks. To clarify, there is no agriculture within the interior of the building. Floors above the 100th level are physically incomplete and are often open to the air where walls or the floor above has fallen away. Avian beasts can be found roosting there.
Agriculture was done on the lands surrounding the arcology. One might deduce that the decline of whatever civilization that lived here was due to the land turning from fertile farm lands to sand-sea, perhaps due to a cataclysmic event or over a long period of time. At this time, the base of the arcology is surrounded by dense jungle, in stark contrast to the desert around it. It is unknown how this jungle survives in the middle of the desert, though one can reasonably guess that the arcology is responsible. In reality, there is an irrigation system beneath the jungle that is still functioning, providing ample water to the jungle. It is a part of the arcology, put in place to create or maintain farmable land directly around the building, probably in response to the climate change. The water is drawn from deep in the earth. Hexes 56.62, 56.63, 57.62, 57.63, 67.64, 58.62, and 58.63 are all jungle hexes.
A primitive humanoid culture occupies the arcology. They find enough food in the jungle and immediate surrounding desert to sustain a population of several hundred. They are very protective of their building, as nomad tribes have tried to take it from them in the past. They attack intruders on sight. However, should one gain their trust, they might tell them that they have lived in the building for as long as anyone could remember. They call it the Pillar of the World. They claim their goddess built it for them in the beginning of time, but she smote the building when a dark and evil god took control. She left the building intact as a reminder of her works that she was forced to destroy. There are tales that once there were many such buildings taken over by the dark god, but only one was left standing by the goddess. The humanoids consider the top of the building to be the most sacred space, consisting of but a few hundred square feet of accessible space that is open to the air. Due to the height of the building, the local tribe has mastered glided flight, and keeps numerous gliders in the upper levels with which they travel between levels and great distances, as well as patrol their territory.
Encounters within the arcology might include primitive warriors wielding metal weapons left over from the old culture. Dogs and primates are kept as pets and guard animals. Above the 100th level when floors are open to the air, abnormally large sized eagles and vultures roost and will defend their nests. There are no constructed traps within the building, however, there are floors and walls that may collapse if disturbed. The advanced and alien nature of the construction makes it difficult to determine what is structurally unsound. The locals are knowledgeable about what to avoid. There are also doors that may be hidden behind debris or obscured by damage, but still accessible, leading into previously unexplored spaces, even by the local occupants. These obstacles and hidden doors should be treated as traditional traps and hidden doors.
Lost artifacts of technology may be recovered from unexplored areas of the arcology. On the second to highest level of the building, a temple has been created by the locals. Magic men and shamans live there as well as numerous guards. The highest level above the temple is a treasure hoard, containing all the precious metals and artifacts owned by the local tribe. It is also where the magic men and shamans go to pray to their goddess.
There are also secret sub-floor levels. The building has 50 sub-floors that have been effectively lost to the local population. They know of a secret entrance, but stay away. Strange things occur beneath the surface and many have been lost trying to plumb its depths.
The corpse of an old and alien being rests deep under the arcology. Ghosts and apparitions haunt the catacombs that were once the service center of the building. The threat of death is minor compared to the madness the residual effects the being has on anything the ventures below the surface. Players risk their very sanity. If players decide to venture down, I would secretly apply to them Sanity Points via the d20 SRD or the Call of Cthulhu game. Players would encounter horrifying images and psychic attacks, all emanating from the dead alien corpse, that deplete sanity points. Ghost and ghouls of dead adventurers roam the halls, often taking on the appearance of victims of inhuman deformities and torture, again depleting sanity. Should the players make it to the bottom and the alien corpse, they would confront a shadow of the being's original form, a sort of demi-god or extremely powerful ghost. There are riches galore in the basement, piled there by the followers of the "old god" before it was struck down. This would be a high-level adventure.