Learn a new thing every day :) Thank you for that.
I wonder if you could pull this off in the modern world. It would be amazing to see sheer walls of granite, completely fused together as one solid mass.
>The expert consensus explains vitrified forts as the product of deliberate destruction either following the capture of the site by an enemy force or by the occupants at the end of its active life as an act of ritual closure.[6] The process has no chronological significance and is found during both Iron Age and early medieval forts in Scotland.
The only way I can imagine is building a clay/mud oven around the entire wall and then having people pump massive bellows for an entire day.