Northwatch Castle
From Drunkapedia
A newly discovered castle in the northern area of dereth, north of Neydissa Castle. You must be 180+ to enter the black market portals found there.
Route
Arwic [33.5N, 56.8E] >> Arwic to Dagger Vale Portal [32.9N, 59.4E] >> 81.5N, 25E