Page "Chess960" Paragraph 24

# Unimpeded: All the squares between the king's initial and final squares ( including the final square ), and all of the squares between the rook's initial and final squares ( including the final square ), must be vacant except for the king and castling rook.

( An equivalent way of stating this is: the smallest back rank interval containing the king, the castling rook, and their destination squares, contains no pieces other than the king and castling rook.