Meisterwerke

Suppose white may still castle. The whole position unlocks when black can uncapture axb. The only way this can be done is if the black rook goes to d2, and the white knight from d2 unpromotes on a8, and goes to a3 or a2. Retro-play would be:

-1. ... Rb7-c7 -2. Ba2-b1 Ra7-b7 -3. Bb1-a2 Ra1-a7 -4. Ba2-b1 Rc1-a1 -5. Bb1-a2 Rc2-c1 -6. Ba2-b1 b6-b5 -7. Nb1-d2 Rd2-c2 -8. Na3-b1 b7-b6 -9. Nb5-a3 d6-d5 -10. Nc7-b5 e6-e5 -11. Na8-c7 e7-e6 -12. a7-a8=N f6-f5 -13. a6-a7 f7-f6 -14. a5-a6 g6-g5 -15. a4-a5

and black is out of retro-moves, since both d7-d6 and g7-g6 must have been played earlier to let out the bishops. So unlocking the position, assuming white may still castle, fails on 1 tempo.

So white may not castle. Unlocking the position is then easy. The white king came from f1, and the knight can unpromote on a8 without problems then.

h#1 with 1. Ba2 Nd3#

Solution by Philippe Schnoebelen