Answer by
coolnoob1 (25)
It's called castling. If neither the king nor the rook have moved from their original positions, and there are no pieces between them, you can move the king two spaces over, then move the rook next to it toward the center of the board.
Another condition is that no spaces between them are under attack (i.e. not only neither piece can be under direct threat but also no space in the path of either piece can be under direct threat) add a comment