The Verum Transitus resides within a castle on the world of Erebusa within Void Space. The castle itself houses an extra-dimensional pocket realm which contains endless passageways and tunnels, both above and below ground. The castle itself is ancient and timeless, the interior of the castle is empty and without any living entity at all. It is also known as The Crossroads. Upon entering the castle from the outside, one finds themselves in a central great hall, from which all of its staircases and hallways originate.
Within the castle hallways branch and turn infinitely, each one leading eventually to a different point in the multiverse. One has to know how to navigate the castle in order to get to the point in time, space, and reality they wish to go. Each branch of the hallway is marked by a different series of arcane runes, these runes form a guide though their original language has been lost to time. The runes allow a person to navigate the endless hallways, staircases, and pathways.
When one knows where they wish to go, the path becomes clearer and the hallways shift and move before you to make your path easier and quicker. This effect works in reverse, upon arriving within the crossroads, one quickly finds the path away from the chamber leads back for the great hall. From the great hall a person is able to follow various paths to other places in the multiverse.
Teleportation
The castle does not contain portals exactly, but rather hallways end in circular rooms with a crystal at the center of the room, the walls with bands of arcane runes. Once inside the room, the crystal is activated by touch where it then teleports the person through the networks of connected realities to their destination. Transit to the Verum Transitus is accomplished much the same way, there are sites throughout the worlds in the realities which act as natural conduits, but one has to know the secrets to activating those sites.
The secret to activating one of these sites lies in knowing the reality you are in, the arcane runes within the Verum Transitus which guide a person to each reality form the runes which are used to call upon the site to teleport you into the Verum Transitus itself. The runes each have a pronunciation, though the language has long since been lost, they can be spoke in this way as arcane words or drawn upon parchment, which acts as a portal key. Once used such a key vanishes.
The Under Realms
Paths within the crossroads can also lead to places not within the material plane, these are accessed via the underground passageways, they work much the same way but are darker and far more foreboding. These pathways are far less friendly to visitors and one must know exactly where they are going or they risk being lost forever.