White retracts his last move and #2

[gritty grail]

Retract: ||0-0-0||
Play: ||axb3 (any) Ra8#||
?

[outer sequoia]

||that only works if black can't castle - and I don't think you can prove that they can't because the rook on g5 could be the Queen's Rook and the DSB could have been captured on f8 by a knight hopping in and out again||

[lone geode]

Hint: ||How did d5 bishop get there?||

[somber summit]

||What kind of insane position is this? The only way to get the white bishop promoted without overcounting the possible amount of pieces taken is to take from f7 to g8 or e8 with the h-pawn and capture e6 with the d-pawn. This also means that no other black pieces have been taken The black f-pawn took 2 pieces to get to h5, and two more pieces at least were taken for the a and b pawns to transition to the b and c files. This accounts for 4 pieces, and since two are left, one white piece is unaccounted for and is possibly captured normally. The black king probably moved to e8 some time after the bishop went to d5, or at most one move before as Bf7 and a bishop moving from e8 will both control e8. This also means that it can be proven that black cannot castle in this position., which does indeed justify Anon581's move. However, retracted moves can include Bd5 (only from certain squares), Rd1 or O-O-OThis leaves us with 3 possibilities. Considering, however, that this is a puzzle, only one of the possibilities seems like a logical retracted move, which would be a retracted O-O-O and instead axb3.||

[lone geode]

Great! Correct, except for one thing: ||The reason that black's 0-0 is illegal is that the white pawn which promoted to bishop had to step into f7 or d7, depriving white king from castling rights. Your reasoning does not work because promotion on e8 isn't forced.||

[somber summit]

Yeah, that is what I meant