Portals were permanent teleportation effects that linked places between planes. A portal was a two-dimensional circle, but not limited in size or shape, stretched across an area that was both intangible and invisible. Due to this, a freestanding archway of some kind usually was built to identify the location of a portal, so those who could not detect magical auras did not stumble into them accidentally. It was impossible to visually find out a portal's destination. For example, sticking one's head into that field to find out the destination was impossible, for touching a portal activated it.
Portals were generally employed as magical transportation devices. When something touched the two-dimensional field, it was affected by a greater teleport effect towards a pre-set destination. A portal could only send a person to its destination, but not back. Quite often, a second portal was built at the other destination to effectively create a two-way-portal.
Destroying a portal was very difficult as disintegration, physical assault, or other physical means could not damage one. Anti-magic Spells could stop the working of smaller portals, but often only temporarily halted the effects of greater ones.
Many portals had specific requirements for activation. Some only allowed the passing of creatures or organic material through, some required specific items or keys, and some only allowed nonliving material. The only universal trait between all portals was the the "limit" of magic that could pass through them: incredibly magical beings, such as Archfey or Dark Powers, were too infused with energy to travel through them.