Portal

Portals are a necessity for the movement of people from one place to another. Most teleportation spells require knowledge about the appearance of a place, but a Portal is a static doorway from one place to another and so needs no such knowledge. They are especially useful for the movement of mages from one planet to another, as no other way exists to do this.
Portals are created from relatively complex rituals, so usually only Sanctuaries, magical colleges, and Limbo have static Portals. Many factors have to be considered when erecting a Portal, including temperature difference, humidity, and height above sea level, as well as geographical location. In Limbo, there is an entire building, the Central Portal Office (or CPO), dedicated to Portal transportation to locales on various magical planets.

Access & Availability

Short-range temporary Portals such as between two landmarks of a city or between short distances in the countryside are a staple of travel for Master mages, but larger or more permanent ones are closely guarded for obvious reasons.

Complexity

Anyone with a significant grasp of Time magic can build a small and incredibly temporary Portal, but larger portals require Rituals and material outlay to function properly.