It was serialised weekly in five parts in The Saturday Evening Post in October and November 1931 as "Your Castle of Hohenems", illustrated by F. R.

[2] It was Yates's first adventure novel to not feature Richard Chandos, and introduced new heroes: John Ferrers (who narrates the story), and his cousin Hubert Constable.

Ferrers and Constable inherit the Castle of Hohenems in Austria and seek the priceless carved jewel collection of its former owner, the Borgia Pope Alexander VI, walled up within the dungeons.

The author, living in France at the time, seems to have been homesick, as the dedication reads "To the finest city in the world incomparable LONDON TOWN".

[3] Mercer’s autobiographer AJ Smithers, writing in 1982, suggested that "his public ought to have become disenchanted by now with such well-worn stuff, but it cannot be denied that Safe Custody came as fresh as any of the others".