The Citadel resides within an unusually inaccessible realm. The only possible way to enter the realm is via a portal opened by a Citadel Guardian in the process of apprehending a cosmic criminal. It is nearly impossible to escape this realm due to the strange, spacial physics of the dimension. Cosmic criminals are encased in crystal, their lives preserved, to be incarcerated in this manner for eternity (along with any clothing and items currently in their possession).
Some examples of cosmic level crimes include:
- Genocidal mass decapitation and head eating
- Planet dicing
- Being some kind of evil alien bureaucrat
- Killing a Wish Master
The realm of the Citadel exists outside the conventional fields of space, similar to Prismo's realm. The prison is composed of six massive crystal pillars, focused toward a crystal frame. Each pillar has its own gravitational field but is held in place by the gravity of the realm's absolute center. The pillars also appear to be in unison with the sentient guardians, and systems similar to the guardians' bodies can be seen inside. As of the episode "Escape from the Citadel," the Citadel is unpopulated and in ruins. Much of the prison's surfaces have melted and the remaining liquid gravitated to the center, forming a massive sea. The Guardians have been eradicated by the Lich's plague before the Lich's rebirth. Guardian Blood appears to offer no protection from the Lich's plague, though it does indirectly have the power to destroy the Lich by restoring life to the Lich's undead body. This in turn cures any surviving creatures that may have been afflicted by the plague.
A group of seven cosmic criminals are known to have escaped from the Citadel. The group included Finn's father Martin, who assumes leadership of the group, as well as the Void Caster and five other "Ancients." The escape was perpetrated after the destruction of the last Citadel Guardian by riding a crystal shard through a portal leading to the vacuum of space (conveniently opened by the Void Caster). Their survival is presently unknown, though plausible.
In "Wake Up" and "Escape from the Citadel," the Lich commits a Cosmic Crime by killing the wish master Prismo and is taken to the Crystal Citadel, where he releases all of the dangerous criminals. The Citadel is razed by the Lich, as the Guardians' ability to destroy life could not harm the Lich. Finn and Jake are indirectly responsible for the destruction, by giving the Lich the opportunity to continue his purpose.
- It has only ever been referred to as "The Citadel" during the currently aired episodes.
- The Crystal Citadel was initially described somewhat differently in the storyboard for "The Lich" (pages 142 to 145).
- According to the storyboard, Finn's real dad was unlawfully imprisoned in the Crystal Citadel where he battles the criminals within to this day. (Apparently this plot line has been dropped.)