You can't castle, While you are in check, You can't castle if you have moved your king & or the rook, in which side you plan on castling to. You can't castle if you king will pass through a check.
Did you put your opponent in check and he/she blocked with another piece, If so he is still right to castle.
I am playing a game on chess.com. I have had my opponent in check and he has subsequently castled. I believe you can't castle after being in check. I thought the compter would prevent his move, do I have to challenge him?