The lowest layer of the stratosphere, twenty kilometers up. That was where Francesca’s workshop was.
It was a giant airship, two hundred meters from end to end. Francesca had taken a high-altitude unmanned airship that the US military was in the process of testing, applied many layers of wards to it — invisibility, wind-aversion, etcetera. She had piled modification on modification, mystically, scientifically, and as a matter of taste. That said, it was hardly a mobile fortress, bristling with weapons straight out of a scifi novel. It was simply a two hundred meter balloon used to lift the meager area of Prelati’s workshop.

Fate/Strange Fake: Volume 4, Chapter 12 ("Day 2, Morning")