Thanks for some REALLY great ideas, guys.
I am the GM.
I was considering "winging it" without the write up since these are plot devices that I don't expect to have used in combat. But just tossing the game effects around helps me plan even that stuff.
I am allowing them to be shut down, with a simple spell or physical disruption. (Think Star Trek:TOS Transporter).
I'm thinking of placing 3 or 4 per continent, or 1 per kingdom (whichever is greater).
While the ancients that made them are still mysterious, the portals still see use. (I'm basing this world roughly on Aaron Allston's "Ean"{The Blood 'Home' World}, so nearly all royalty can 'Port).
I forgot to mention: I'd like a "Gate" advantage too.
I'd like the King to go up, activate the portal, and walk his entourage of diplomats through.