The stickler for detail in me (i.e. nerd with too much time on his hands), noted that in the description of the main fortress, it is described as being carved of “solid black gneiss” rock. That should probably read ‘igneous rock’. Gneiss rocks are striped or banded, not solid. Although, black and gray banded gneiss rock with glittering flicks of mica and crystal sounds pretty cool too.
From the very beginning, I thought Gary was a master at descriptive text, mood, sensory details, etc. D3 is amazing in that regard. So is Tsojcanth. Dialogue was a skill he developed later, but he did develop it.