in der Retroanalyse
19 - Dr. Niels Høeg
Deutsche Wochenschach, 1907
14+11. (RV) Matt in 3 Zügen
If black still can do both castlings, his last move must've been d7-d5
or f7-f5, since a pawn capture on d5 or f5 would take too many captures.
Now suppose black can't castle queenside. Then #3 with 1. Ng6!
The last case is: Black can't castle kingside. Then #3 with 1. Nc6!.
Solution by Philippe Schnoebelen