Here it is. In post, outside the playable map in "Old Ascalon", south-west of Sardelac, is the entrance to the Catacombs. In Pre you can exit Foibles Fair and it's a short walk north-northwest, past a few bears and bandits. It also has an active loadzone bar underneath it.
That's really neat! I'm so curious about what would happen if you somehow managed to pass through those portals.