He Who Whispers (Dr. Gideon Fell, #16)

At the edge of the woods by the river stands the tower. Once part of a chateau since burnt down, only the tower remains. The inside is but a shell with a stone staircase climbing spirally up the wall to a flat stone roof with a parapet.
On that parapet the body of Howard Brooke lay bleeding. The murderer, when Brooke's back was turned, must have drawn the sword-cane from its sheath and run him through the body. And this must have occurred between ten minutes to four and five minutes past four, when the two children discovered him dying.
Yet the evidence showed conclusively that during this time not a living soul came near him.