Teleportation circles were permanent Portals created by conjuration Spells, allowing travel anywhere in the same Realm. The teleportation circle appeared on the floor around the caster, or on whatever horizontal surface they stood on, with a diameter of 10 feet. Any creature who stood within the circle was teleported to a designated location that could only be set once. If the caster tried to designate a location that was a solid object, an area they were unfamiliar with and lacked a clear description, or was in another plane, the spell failed; failed teleportation circles were incredibly dangerous, as they could tear apart any creature who walked inside.
The circles were discreet, and very difficult to spot, so most casters marked the circles in some way to keep creatures from activating it accidentally.