Verity's ambition soared farther.

Very high up on the wall, above the glorious pillars, and just under the clerestory windows, was a narrow passage called the Nuns' Ambulatory.

It had been built in the long-ago ages to provide exercise for the sisters in the adjoining convent, to which a covered way had originally led.
"Just think of the poor dears parading round there on wet days when they couldn't walk in their own garden!" said Verity, turning her head almost upside down in her efforts to scan the passage.

"I wonder if they ever felt giddy." "There's a balustrade, of course, but I prefer our modern gym.

I believe there's a walk all over the roof too.