In Book the Third, Chapter VIII, of A Tale of Two Cities who is a turnkey?
In this chapter, Miss Pross and Jerry Cruncher shop for groceries for Lucie and her child and themselves. While they are out, Solomon, Miss Pross's long-lost brother appears; however, he is knownas John Barsad by Jerry Cruncher who identifies him as the spy who accused Charles Darnay of treasonous acts against the crown of England. And, while Barsad is engaged in conversation with Cruncher, Sydney Carton, too, appears and recognizes Miss Pross's Solomon as the witness in the earlier trial of Darnay. Also, Carton realizes that Barsad is a turnkey, or jailer in charge of the keys, at the Conciergerie.
That Barsad is a turnkey at the prison where Darnay is being held is a boon to Sydney Carton. For, now, he can coerce the spy by "playing his hand" and obtaining through Barsad passage into the prison. Addressing Solomon/Barsad, Carton says,
"Mr. Barsad,...Sheep of the prisons, emissary of Republican committees, now turnkey, now prisoner, always spy and secret informer, so much the more valuable here for being English that an Englishman is less open to suspicion of subordination in those characters than a Frenchman represents himself to his employers under a false name. That's a very good card, Mr. Barsad, now in the employ of a republican French government was formerly in the employ of the aristocratic English traitor and agent of all mischief so much spoken of and so difficult to find
"I play my Ace, Denunciation of Mr. Barsad to the nearest Section Committee. Look over your hand, Mr. Barsad, and what you have. Don't hurry."
Thus, Barsad's position as turnkey is pivotal to the plot at this point in Dickens's novel. For, with his knowledge of Barsad, Carton is able to effect his plan for saving Charles Darnay and redeeming his dissolute soul.