Mark Thompson wrote on Sun, Jun 29, 2003 12:12 PM UTC:
'Wouldn't it be, in fact, impossible to move the rook 'through' an
attacked position? Since the only position the rook goes 'through' is
the one position where the King is going to end up.'
Not when you're castling Queenside. Then the King ends the move on square
c1, the Rook on d1. The Rook passes over b1 and the King does not. The
King always moves two squares when castling to either side.