Requirements
Castling is only permissible if all of the following conditions hold:
• You must never have moved your king.
• You must never have moved the rook you want to castle with.
• There must be no pieces between the king and the rook.
• The king and the rook must be on the same row (known as rank in chess).
• The king may not pass through squares that are under attack by enemy pieces, and the king cannot be in check after castling.
• You can not castle if the king is currently in check. However, the king may have been in check previously, as long as it isn't in check when you want to castle.