|“||The walls of the Adamant Citadel are like the lives of the Iron Sisters themselves: hard, unyielding, and strong. Their motto, and the motto of the Citadel, makes this clear: ignis aurum probat. "Fire tests gold."||”|
Even more hidden than the Silent City, the Citadel's location remains unknown, despite its age. It is reached through one of a number of entrances scattered around the world, the oldest of which is found on the lowest floor of the Armory in Alicante. The Portals lead to the volcanic plain of the Citadel, outside the fortress walls.
The volcanic activity of the land it rests upon serves as a convenient defense for the Citadel, as well as provides the Sisters with the extreme heat required for their forges. The lava also serves as a disposal for weapons that can no longer be used, lest the adamas get into the wrong hands.
The actual Citadel building can be reached only by crossing a drawbridge. To enter, the female Shadowhunter must cut her palm, draw blood into the chasm separating the fortress from the plain, and recite the motto of the Iron Sisters, "Ignis aurum probat." This causes a flash of blue light and the drawbridge to lower.
The Citadel is seemingly just a single fortress, its depths however unknown. The Adamant Citadel stands on a volcanic plain, a stretch of moss and dried lava beds ringed by a narrow river of molten lava like a moat. A ring of tall, smooth, unbroken and a seemingly continuous band of adamas, with no sign of mortaring, also surrounds the Citadel.
The fortress towers high over the plain itself, with a ring of towers around it. The entire structure is carved out almost entirely of adamas, giving off a gentle white-silver light. Its towers are tipped with glittering electrum. The whole fortress is surrounded by a high wall.
Set in the walls is a single gate, formed of two gigantic blades plunged into the ground at angles, crossing each other. Separating the gate and the top of the ridge plain is a vast chasm that opens into the river of lava that encircles the fortress. Connecting them is a closed drawbridge that only lowers upon the sacrifice of female Nephilim blood. The drawbridge is strewn with knives at random intervals, embedded with their blades pointing upwards, making it impossible to approach the Citadel in haste or without care.
Its inner chambers may be walked only by the Sisters themselves. Although they accept female visitors, they are welcomed only into the receiving antechamber, the only room outsiders are permitted to see. The whole antechamber also dimly glows with adamas. Carved into the center of the floor is a black circle with the sigil of the Iron Sisters: a heart pierced by a blade. The room is simple, with no furnishings or comforts.
The earliest incarnation of the Adamant Citadel was constructed by Abigail Shadowhunter, the first Iron Sister, and six other Nephilim. In the early days of the few iron Sisters, the Citadel was merely a single forge and a set of wards, though the paths leading to the volcanic plains where it is situated are not as hidden and guarded as it is now.
In the early 1900s, for her crimes, Tatiana Blackthorn was sentenced to live with the Iron Sisters. Before she left, the Prince of Hell Belial explained that it had been his plan all along for her to be sent to the Adamant Citadel so that she could attack the Clave from within.
In 2007, Sebastian Morgenstern and his Endarkened army attacked the Citadel, using the attack to lure Shadowhunters out of Idris and ambush them. Sebastian's men also tried to enter the fortress but were killed by the traps before they could even cross the bridge. While normally inaccessible to others, they were able to enter the area of the Citadel because it was built on ley lines—the province of the fey, who were then in league with Sebastian.
- Jocelyn Fray and Isabelle Lightwood, with the help of Magnus Bane, reached the Citadel through a door in the old Augustinian Monastery on Grymes Hill, in Staten Island. It is said to be the entrance closest to Brooklyn.
- According to Lucie Herondale, the Citadel is located in Iceland.