The rule that makes it simple
In Chess960, castling ends on the same final squares as standard chess. That means: kingside → king on g-file, rook on f-file; queenside → king on c-file, rook on d-file.
The checklist (conditions)
- King and rook have not moved.
- All squares between them are empty.
- The king is not in check, does not cross check, and does not end in check.
Use the helper
Use the checklist tool: Chess960 Castling Helper
If you’re new to the variant: What is Chess960?