The Judas window : a Sir Henry Merrivale mystery

"Jimmy Answell is summoned for an audience with Avory Hume. The two men are later discovered after witnesses break into Hume's study - a room with bolted steel shutters and a heavy door locked on the inside. Answell is found lying unconscious and Hume stabbed to death with an arrow. How can young Answell but be guilty? How could Sir Henry Merrivale (H.M.!) be foolhardy enough to undertake his defence at the Old Bailey? And what is the `Judas Window' to which H.M. keeps alluding?"
""Only young James Answell could have committed the murder. After all, he was found unconscious in the locked room next to the body of the murdered man. His clothes were disheveled from an apparent struggle. The whiskey decanter containing the liquor he said was used to knock him out was full to the brim. All the glasses on the table were clean. His fingerprints were found on the murder weapon, an arrow from the victim's collection. Furthermore, he was heard arguing with the dead man, whose daughter he wished to marry. Just about everyone is convinced that James is headed for a date with the hangman. Everyone except Sir Henry Merrivale, H.M. to his friends and associates. He's convinced that the real murderer used a 'Judas window' to commit the crime."--Page 4 of cover."@en