2019-10-18, 09:16 AM (ISO 8601)
Re: Teleportation Circles - Permanence
Ultimately, how a teleportation circle is used in game is up to the DM. They are free to play it however they want. In their world, casting a teleportation every day for a year could create a permanent link to another location.
However, the only practical issue with that is that eventually the world would be flooded with permanent links. Consider a trading cartel that ships goods from Baldurs Gate to Waterdeep or to Port Nyanzaru in Chult or to Mulmaster. How much do these companies spend on ships, guards, food, or labour getting their goods from one location to another? In a world with permanent teleportation circles that are always active, every single company would be vying to acquire the ~36,000gp required to establish a permanent bi-directional link for their private use. It has a large cost over a period of a year but you won't be paying mercenaries, guards and many other expenses. In the DMG, a galley costs 30,000gp. Instead of a fleet of galleys, the trading cartel would set up a teleportation hub with links to its trading partners. It is safe, instantaneous, not vulnerable to pirates or weather, and only costs ~18,000 gold each way. They would also invent containerized shipping so that all their goods could be quickly and easily moved through the permanent links.
There would be lots of unemployment of sailors at least initially. The only need for ships would be warfare and exploration. Rich people would have their own permanent links to their work, their keep in the country, and their escape route out of town in case of attack. In addition, since the links are "permanent", they would stick around forever unless someone destroyed the sigils so unless teleportation circles are new there are likely old pre-existing teleportation circle networks from previous civilizations.
I think that, in terms of "intended", the repeated use of the spell is just meant to create a permanent SIGIL that can be used as a destination for the teleportation circle spell. The text says that when used in this way, it doesn't need to be used for transportation. Since teleportation circle requires a 9th level caster and the teleportation spell a 13th level caster, this approach makes it convenient though expensive for large organizations to have a permanent and error free teleportation destination available to those who know the "address" without the extreme disruption to the economy and society that would follow logically from the establishment of actual permanent links (no matter how cool they are :) ).
However, as mentioned, every DM is free to run it in whatever way they like, there isn't a "correct" interpretation. Reading the RAW depends on how you interpret "permanent teleportation circle".
The first sentence of the spell says "As you cast the spell, you draw a 10-foot-diameter circle on the ground inscribed with sigils that link your location to a permanent teleportation circle of your choice whose sigil sequence you know and that is on the same plane of existence as you."
The target for the teleportation spell when used for transportation is a "permanent" teleportation circle whose sigil you know. A DM could rule that a permanent teleportation circle can either be a source or a destination circle, meaning that the permanent teleportation circle can either contain a permanent link to another teleportation circle or it can just be a set of sigils that can be used as a destination for a teleportation circle spell. This is required since without the differentiation it is impossible to create the first teleportation circle since it would have no target sigil to link to and so could not be used for a permanent link.
Anyway, I think there is enough content in the spell to interpret it as the "permanent" teleportation circle being intended to inscribe a permanent and unique set of sigils that can then subsequently be used to target teleportation spells so they can be cast without error and not to create a permanent link but it is up to the DM in the end :)