Castling has specific requirements involving the king, rook, occupied squares and attacked squares. A rook being attacked is not the same question as the king crossing an attacked square.
What to do
Check that the relevant king and rook have not moved, the required spaces are clear and the king is not in check. Verify the king’s transit and destination squares under the applicable rules.
A mistake to avoid
A king cannot castle out of check. Do not infer legality merely because the king’s destination looks safe.
A detail worth checking
Check attacked squares in the position before the castle. A blocked line can change when pieces move, so evaluate the rule’s actual position rather than a remembered diagram that only looks similar.
Try this practical check
Set up three legal training positions that differ only in an attacked king-path square. Explain the decision before checking the rules reference.
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