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....
After the PCs defeat (or force the rapid retreat of) a villanous necromancer/demon summoner, they discover a book. This book outlines how to summon a demon whose power increases according to the size of the summoning circle used to summon it. And after the PCs examine a map of the country, they find that the layout of the cities and roads match up with the required summoning circle. In fact, the final road that would complete the circle is currently in construction.
With anyone who can find out about the circle and the ritual to summon the demon able to somehow make the final road/line, and destruction of the cities and roads currently in existence out of the question (unless this is an evil campaign- that removes the moral quandary), how are the PCs going to solve this problem?