They lie to the north of Gatekeeper mountain. The mountains can be seen outside the Study of Castle Shadowgate. Furthermore from the Balcony and Lookout they are seen to be fairly close to the castle surrounding to the north and northwest.
The mountains don't seem to appear in Shadowgate 64, instead there are plains surrounding the base of Gatekeeper mountain, and the walls, and four towers that had been built surrounding it. Clouds obscure most the view beyond the walls.
In Shadowgate (2014) the mountains have been moved far west of the Gatekeeper mountains and Gatekeeper Mountain. It lies between the Southern Arm to the west, and the Dragonfel to the east. The Spires of Adgor instead lie to the north. The Darken Range doesn't appear to be nearly as prominent or important as they were in the original game.
The Gatekeeper Mountains can be seen surrounding Castle Shadowgate in the distance, from the peak of Gatekeeper Mountain itself (rather than the Darken Mountains).