Also known as a magnetic fusion drive, or a mega-ion drive.
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.