|Notable inhabitants|| Martin (formerly)|
Void Caster (formerly)
The Lich (formerly)
Citadel Guardians (formerly)
The Crystal Citadel is a universe that has held the most dangerous criminals of the Multiverse for eons. It is protected by the four Citadel Guardians. It was not mentioned or talked about in the episode "The Lich," but it is a possibly unproduced place that was talked about in the storyboard for it.
It is difficult to enter—one must commit a cosmic crime—and nearly impossible to escape due to the strange, spacial physics of the dimension. It is also one of the most accessed universes. 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, and some of the Citadel's prisoners have escaped.
According to the storyboard, Finn's real dad was unlawfully imprisoned in the Crystal Citadel where he battles the criminals within to this day.
In "Wake Up" and "Escape from the Citadel," the Lich commits a Cosmic Crime by killing Prismo the wish master 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 its purpose.