The Judas Window by John Dickson Carr (as Carter Dickson), a Sir Henry Merrivale mystery.

One of the five best locked room mysteries, as selected by 14 established mystery authors and critics (All But Impossible!, 1981. ed. E. Hoch).

The Case: Avory Hume is found dead with an arrow through his heart--in a study with bolted steel shutters and a heavy door locked from the inside. In the same room James Caplon Answell lies unconscious, his clothes disordered as though from a struggle.
The Attorney for the Defense: That gruff and grumbling old sleuth, Sir Henry Merrivale, who proves himself superb in court--even though his gown does tear with a rending noise as he rises majestically to open the case.

The Action: Before H.M. can begin his defense, Answell, his client, rises and cries out that he is guilty. Sir Henry doesn't believe it. But proof, circumstantial evidence, and the man's own confession point to his guilt. So the great, explosive detective gets down to serious sleuthing and at last startles the crowd in the Old Bailey with a reconstruction of the crime along logical, convincing lines.

The Judas Window. Also published as The Crossbow Murder.

Included is the floor plan found in the print version, redrawn for better legibility specifically for this edition.