4 ideas
A wealthy industrialist, Kozaburo Hamamoto, builds a deliberately slanted mansion on a snowbound cape at Hokkaido's northern tip and invites guests for Christmas. A chauffeur is found stabbed in a locked room in a bizarre posture, a second man dies in another sealed room, and the police are baffled until the eccentric detective Kiyoshi Mitarai arrives late and shows that the house itself was the instrument of the crimes.
The impossible crime is solved by recognizing that the architecture itself was designed as the mechanism. The tilt, the layout, and the openings between rooms were built long in advance so that gravity and geometry would do the killer's work. Premeditation can be embedded in infrastructure years before the act, so investigating only the people in the room and the minutes before the crime misses the actual cause.
When an anomaly is framed as personality or quirk, observers file it under explained and stop asking what it does. The most visible oddity can therefore be the best hiding place for the crucial clue.
Before the solution, the narrative stops and formally tells readers that they now hold every clue needed to deduce the truth. This turns the story into a solvable puzzle with a binding contract: nothing essential may be withheld, and the answer must follow logically from the stated facts. It is a core device of shin-honkaku, the revival of puzzle-first detective fiction that ranks logical ingenuity above social realism.