You will know that you are near the Dwarven king's castle when you meet (in Taron mines) a black dragon patrolling a small square-like area in the mines. The castle (Taron) is just upstairs past the portal, in Ardan's Peaks. On the Ardan's Peaks map, castle Taron is right in the middle of the map.
http://smg.photobucket.com/albums/v2...ent=TaronR.jpg