If it was a castle, it was the strangest one he had ever seen.
He of course saw the main tower - taller than anything he had seen outside of Stoneholt, the spire looked fragile and was topped by a glassed-in chamber.
The outer wall was so gently sloped that it would have only stopped a horde of hobbling old men, an able-bodied soldier could stride up to its crest with little effort. Within one saw a huge, nearly perfect bowl-shaped area with the base of the tower in the centre, covered in hundreds of mirrors.
This structure is a massive solar collector designed by the Wizard-King Aardwal in centuries past. He used the concentrated light in his investigations into the magic of light, and in the fashioning of flash crystals.
The runoff from Mount "Evil Volcano" has turned an area of small lakes below into a vision of hell. The mixture of acids in the lakes is of such strength that virtually any organic material touching is quickly dissolved. It is a melange of sulfuric, nitric and hydrochloric acids.
However, the place does harbor life, and its very strange, and very dangerous....
The PCs are setting up camp at the end of their first day of walking through the Esh-Inel Mountains, when they hear in the distance a low rumbling, like thunder. It grows progressively louder until it echoes off the mountainous bowls around them, then dies away again. At its worst the ground starts trembling. It happens every day at this time, and it is the noise of the Great Carts on the dwarfish underways returning home after a day mining, laden with many tons of ore.