First, the three retro puzzles on Good Friday:
1. Black's last move must have been a capture on a3, as Black must be on move (1. a3-a4 is impossible as the Black king must have stood on b4). But which piece? Queen or bishop would have meant the Black king was in an illegal check, and there is no way for the piece to be a white rook, as then White would have no last move himself. That leaves only a knight, so 1. ..Kb4 x Na3 is the only possible last move.
2. It cannot be Black to move, because any king move or Bb2-c1 would have been from illegal positions. That leaves White to move and he must have captured on b1 - but what? (It must be a capture, not a simple move, as there is no legal previous move for Black other than moving a piece to b1). Queen or rook would have been illegal checks (how did they get there?) and a Black bishop could not have got through the pawns. So the only possible last move is Ba2 x Nb1.
b2, a4, c3, a2, c1, d3, b4, c2, d4, b3, d2, b1, a3, finishing up on c4. I suspect this may not be the only solution. Can anyone find another one?