James Jacobs wrote:
The basic way the barrier works (which has been heavilly revised from our first stab at it in "The Demon Within"—the Demonscope and Goriath, if they still exist, are likely quite changed) is that there are a series of wardstones that run along the Mendev, Numeria, and Ustalav borders along the Worldwound, sort of creating an arc of wardstones along the eastern and southern border of the Worldwound.
These wardstones focus their energies into the Worldwound, creating a sort of "sheet" of energy over the region that has a hard border along the east and south sides, and a soft border to the west and north—since those regions have a lot more uninhabited areas, the crusaders were satisfied with saving the time and effort of completely enclosing the Worldwound with wardstones (something that time and resources wouldn't have let them do anyway).
The wardstones effectively create a field along the southern and eastern "hard" borders that works similar to that of a forbiddance spell in the form of a 300-foot-long path that bars teleportation and damages demons in the area each round. This prevents invasions and enforces a no-man's-land strip.
But the big thing these wardstones do is they blanket the Worldwound with an effect that prevents demons from teleporting into our out of the Worldwound. They can still teleport inside the zone, but not out of it.
Demons can smuggle themselves out of the Worldwound if they're tricky, but not on anything near to approaching a mass scale.
There are other effects and things going on as well, but that's the basic way the barrier works. (And to answer the original poster's question—the barrier is indeed invisible.)
More information about wardstones appears on page 301 of the Inner Sea World Guide.
And yes... we WILL be speaking more about the Worldwound someday in the future. Stay tuned!
I hope this means that this will be the first AP to use the the slow progression and filled with so much info that it has to take a full 12 issues to do this conflict justice.