Citadel (Doom Eternal)

The Citadel refers to a number of locations in Doom Eternal.

Hell on Earth Citadel
The Doom Slayer gets on board the Citadel in San Andreas Chasm while traveling through Hell on Earth level.

Necravol Citadel (Tower of Babel)
The Citadel (aka Tower of Babel/Tower Babel) is a massive Maykr facility in Hell, located at the center of the city of Nekravol. 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 functions as a production line of suffering. 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 is vulnerable to extraction and is bought 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 though 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 is 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

 * The unlockable Trading Card for this location is called the "Tower Babel".
 * Strangely there is a statue honoring Olivia Pierce in one of the lower areas of the Citadel.
 * In concept art and a lithrograph Bethesda gave away, the tower is referred to as the Tower of Babel.