3 ideas
A fair-play mystery obliges the author to show every fact the detective uses before the detective uses it, so the reader could in principle reach the solution first. The puzzle's legitimacy depends on this. A solution that relies on hidden information breaks the contract, however clever it is.
A sealed room with a death inside looks impossible only because observers assume the room's state when they found it is the state it was in when the death happened. The detective's method is to separate what was actually observed from what was inferred, then look for a mechanism, timing shift or false assumption that makes the physical facts consistent. The impossibility sits in the witnesses' reasoning, not in the room.
Bringing relatives together at an isolated manor produces a bounded set of suspects who share long histories, grudges and inheritance stakes. This turns a single crime into a study of family motive. The title invokes the legendary ättestupa, the cliff where the old were said to be sent to their deaths by their own kin, which frames the crime around what families owe their elders and what they might do to be free of them.