I've so far discovered two areas there: one where you go from the portal next to the torture rack (on the right as you enter the maze) to the balcony overlooking it, which also has a ledge that you can throw another character to using telekinesis, though there's not much interesting there, just a skeleton corpse; the other portal opens after curing the historian of his unfortunate affliction which gives access to a second area with an overlook, containing a chest and another skull key.
I'm guessing there are potentially more portals to be found given the extra gargoyles in the maze but so far I haven't found them.
