Project Citadel is a division of the United Nations Tribunal on International Law that specializes in extra-dimensional threats.
The history of Project Citadel goes back to United Nations Security Council Resolution 67A, enacted in the 1950's, which gave the United Nations sole authority to decide residency or sanctuary to those outside of our dimension or universe. This would eventually lead to the creation of UNTIL, but also a specific authority in UNTIL that would handle extradimensional threats.
From its inception, Project Citadel has taken a hardline stance on extradimensional travel, considering those that engage in it to be threats to the world of the Champions. This was so true when it came to the discovery of Multifaria, a world that mirrors that of the Champions but where their Defender is an evil tyrant. In the eyes of project leaders, the existence of Multifaria validated their policies.
While it is not "illegal" to be from another universe, Project Citadel and their operatives consider the existance of those from other realities to be "criminals" until proven otherwise. People from other realities have been detained on that basis alone, with their legal status being determined only after a prolonged legal fight by those who could afford such a fight.
One of their most proficient operatives in the organization is Captain Lynch. A victim himself of an extra-dimensional incursion, Lynch was given a special ability to sense dimensional breaches and to detect those from other realities. With the unanswerable authority behind him, Lynch became judge, jury, and sometimes executioner for many from other realities.
Project Citadel's history is given in "Future's Guardian" #5.