June 4 daily puzzle requires an illegal move in solution

Another thread on here complains about "hackers" using an illegal move (en passant)



..., the square he moves through, or the square that he ends the turn....
the king did not move through b1, by the way if the castle is in "check", you may still castle.

This is a common complaint among new players. For example:

Another thread on here complains about "hackers" using an illegal move (en passant)
You can call an illegal move by a fancy name, but it's still an illegal move.
To people complaining about castling here being illegal - which square that the king passes through is being checked? e1, d1, and c1 (the relevant squares) are all safe. The king and the rook haven't moved yet (we don't have a history but this is a safe assumption given the position). The rook is allowed to pass through a threatened square during castling. I see no violations.
It's quite clever because in a kingside castle the rook and king essentially use the same squares, so the intuition is that the rook can't move through a threatened square because it would imply the king did too. This puzzle rightly points out that castling queenside, b1 can be threatened. Try to learn rather than spouting frustration! Best luck all