Simplify the following text for a Grade 9 native English-speaking person.
Original: Castling consists of moving the king two squares on its first rank toward either one of the original rooks , then moving the rook onto the square over which the king crossed .
Simplified: Castling consists of moving the king two squares on its first rank toward either one of the first rooks , then moving the rook onto the square over which the king crossed .