The Citadel is a large sandstone palace, largely carved from the canyon walls surrounding the Shadow Carja settlement of Sunfall. The design is typical of Carja architecture, being comprised of copper and sandstone, albeit less ornate than what can be seen in Meridian.
Aloy's access to the interior of the Citadel is limited to the throne room and the hallway leading to it: however, the inside appears to be far more opulent than the exterior, even more so than the Palace of the Sun.
From the throne room leads a balcony overlooking the Sun-Ring. From this balcony leads to a vent that allows entry to the main entrance of the Zero Dawn Project Facility.
During an inspection of the garrison by the eleventh Sun-King, the Radiant Marzid, in the sixth year of his reign, the monarch was struck with a vision of an ornate palace and ordered the Sundom's resources to be redirected to building the structure. Following its construction, the Citadel became Marzid's summer palace, a place where he would create many of his paintings. Marzid would eventually die in the Citadel, having been poisoned by the fumes from his paints.
It is unclear how the palace was used during the reign of Sun-King Hivas, but Sun-King Jiran would also come to use the Citadel as a summer palace.