The Styx Citadel is the Styx headquarters, located in the Colony. It is a tall building carved from the stone of the cavern wall. It is well guarded. The Garrison building is located right next to it. The Old Styx is shown to have an office in it. The Citadel is where the Styx orchestrate most of their major operations.
In the series Edit
Will mentions sketching the Citadel in his journal while staying with the Jeromes. He finds that this is difficult; the Styx sentries make it hard for him to get close to it to draw it in detail.
When Sarah Jerome is brought back to the Colony, she stays at the Garrison building right next to the Citadel.
Eddie makes a passing reference that the Styx used to have tunnels linking the Citadel to the Houses of Parliament and their vaults. Guido Fawkes fell for their secret plan in the 1600s (the Gunpowder Plot), which led to the tunnels being sealed off.
While discussing the underground operation including Eddie, Chester asks why Drake doesn't just use explosives and destroy the Citadel. Drake replies with his opinion on collateral damage- he finds it better to know the Styx are down there rather than active all over the country. Later on, in the Colony, Drake remarks how the nerve gas he is pumping in through the Fan Stations will not affect the Citadel (it has its own air supply).
In the final chapters, many Styx are summoned to the Old Styx's office in an upper Styx Citadel room. He rages over Drake's successful plan of killing the Plague Snails and destroying the Laboratories. However, he is calmed when the Rebeccas appear and present him the Dominion phial and news of the New Germanian Army.
The Second Officer finds that the Styx are using Dark Lights on an extensive scale, seeing purple lights from the Garrison and probably the Citadel. Later on, when Will, Drake, and co. re-enter the Colony, they find the Styx have set up subaural fields to prevent people from entering the now-vacant Citadel. Many Colonists have been knocked out by the fields. Joseph the Colonist (a friend of Sarah) explains to Will that, because he is using special earplugs, he can help rescue the knocked out Colonists on the Citadel property.
The Styx Citadel is a rather plain-looking building, carved from the stone itself. It is seven stories high. It protrudes back into the rock for a while, so no one really knows how big it is. A fence surrounds it, with Styx sentries preventing anyone not a Styx from getting close to it. The building has black crystal windows.
The inside is not really described. However, the Old Styx's office floor has whitewashed walls and stone floors, with light orbs hanging from the ceiling (a monastic feel). A pipe-message transport system (like the one in the Hold) is presumably used, as the Scientists were able to send a message to the Old Styx through one.