A King and a Rook may castle everytime and everywhere (even when they
have already moved) if
• both are one the same rank or file
• there are at least two squares between them
• all squares between them are empty
• the King is not in check and he does not move over an observed square.
Castling is done by moving the King two squares towards the Rook and
then moving the Rook to the square behind the King.
14+14. Proof game in 19.5 moves (Rokagogo)
1.f4 a5 2.f5 a4 3.f6 Ra5 4.f?#8212;e7 Re5 5.e?#8212;f8=N Qf6 6.Ng6 h?#8212;g6 7.Nf3
Rh3 8.Nd4 Rhe3 9.Nb5 0-0 10.N1a3 0-0 11.d4 Q?#8212;f1+ 12.Kd2 Qf6 13.Ke1 Qa6
14.0-0 d6 15.Rf6 Nd7 16.Bf4 Nb6 17.Qd2 Na8 18.0-0-0-0-0 b6 19.Qd1 Bb7