The structure of vampire Courts was something very interesting for Cael as a mage. The spells that held the sub-spaces together didn't seem to be attached to anything outside of them, but neither Cael could find anything like magic glyphs on the inside.
It was really something divine in origin—a shelter made by the hand of a god. Vampires themselves needed special artifacts—keys—to enter and exit.
An invitation to a court was mostly just a one-use key.
Together, this meant that no one could leave or enter the court without permission from its Head, or someone with a key. Most vampires had personal keys, but Cael had to actually ask permission from Reiner just so Ginevra and Mimi could leave.
This was easy. The laws of hospitality meant that guests could leave whenever they wanted. Things were a bit more complicated for Cael.