Citadel (Doom Eternal)

Citadel refers to several locations in Doom Eternal.

San Andreas Chasm Citadel
The Citadel (aka as the Citadel of Hell) in the San Andreas Chasm is a facility used as a meeting place on Earth by the Hell Priests, seemingly acting as a command post. The Doom Slayer travels there after killing Deag Nilox to interrupt a meeting of the Hell Priests and confront the Khan Maykr. Little of the interior of this facility is seen during his visit, and it's not clear what other functions, if any, the structure has.

Arctic Citadel
The cursed citadel in the Arctic comprised of the Cultist Base and Doom Hunter Base, with both ruled over by Deag Ranak. The entire region is filled with ruins, and remains of demonic skeletons, of various types including Doom Hunters, and a skull of a Baalgar demon, and what is apparently a Titan skeleton under the sea. These skeletons and ruins appear to be dug up, and used in the research at the citadel. The fortress was built upon the site of ancient Golgothan ruins, an Argenta colony from around 60 million years ago.

Functionality
Collects the remains of Agaddon Hunters and converts them into Doom Hunters. Also, at least certain parts of the Cultist Base appears to be used to turn cultists into Revenants or Carcasses, and create other bio-weapons for Hell Priest's forces.

Nekravol Citadel
The Citadel in Nekravol is a massive Maykr facility in Hell, located at the center of the city. The structure serves as a processing center which sorts, torments, and extracts the souls from its victims in order to produce an endless supply of Argent Energy.

Functionality
The Citadel is one of the Soul Spires. It is, in fact, the largest Soul Spire. It functions as a production line of suffering. Below it is the Soul Factory, where the corpses of mortals killed by the demons are delivered. In the factory, a demon called Kalibas, known by the epithet "the Sightless Judge", is used to select victims, as it seems not all souls are suited for the Citadel's needs. Those who are not selected are discarded into the surrounding Blood Swamps.

Those that remain are tortured without mercy until the soul loses all hope: at this point it's vulnerable to extraction and is brought to a Maykr-designed machine called the Evulsor which rips the soul from the body. The soulless husk is then discarded into Hell's wastelands to become a demon, while the souls, reduced to a raw form called the "aura", are melded with Wraith essence and exposed to the fires of Hell to form Argent Energy. This energy is sent through gates to locations throughout the universe to bring power to those aligned with the Maykr, with a portion sent directly to Urdak.

A side-effect of this procedure is that it vastly accelerates the transformation of the soulless husk into a demon, a process that would otherwise take eons. As a result, the Citadel's byproducts serve to bolster the ranks of the armies of Hell.

History
The facility, one of a number of "Soul Spires" throughout Hell, owes its existence to a deal struck between the Khan Maykr and the Dark Lord, under which Hell would be provided victims to be transformed into demons, while the energy from their extracted souls would be used to sustain the realm of Urdak. Under the terms of this deal, the forces of Hell would not act to prevent the construction of the Soul Spires.

The factory was built from stone and a material called "fell-steel" by Argenta slave labor under the direction of the Argenta priesthood, the Order Deag. This was required as the Maykrs are unable to enter the realm of Hell. Construction of the facilities occurred without the knowledge of Argent D'Nur's protectors, the Night Sentinels, and discovery of this deception was a key factor in the civil war that engulfed Argent D'Nur, ultimately leading to the Order Deag betraying the Sentinels and offering up the Wraiths to be imprisoned in The Well.

The facility was attacked by the Doom Slayer during his response to the invasion of Earth, as the energy transfer point was the only place to enter Urdak. The Slayer destroyed Kalibas and a piece of machinery at the top of the facility as he passed through, though he did not destroy the tower itself.

Trivia

 * Strangely, there is a statue honoring Olivia Pierce in one of the lower areas of the Nekravol Citadel.
 * In concept art and a lithograph Bethesda gave away, the Nekravol tower is referred to as the "Tower of Babel". Its Steam trading card also bears this name.