Originally posted by nickhawker
The white knight on g1 could have come out and the rook could have been taken on this square?
Edit: meant to reply and quote, this was in answer to "why not?"
Replace 'could have' with 'must have'.
Black is on the move, he made an even number of moves, all knight moves, allowing them to go back to their original position.
Hence, white made an odd number of moves, consisting of an even number of knight moves, and an odd (at least one) number of rook moves. The rook on a1 can only have made an even number of moves, hence, the rook on h1 must have made an odd number of moves, and was captured on g1.
The capture was Nh3xRg1 because the knight would have harassed the white king from f3. And the knight must have gone back via h3: Ng1-h3 for the same reason.