Illegal move

Hello, in my last game my opponent castled long, but my bishop was on that diagonal! How it's possible? (MOVE 19)



Thanks

Castling is permissible provided all of the following conditions hold:[4]

The castling must be kingside or queenside.[5]
Neither the king nor the chosen rook has previously moved.
There are no pieces between the king and the chosen rook.
The king is not currently in check.
The king does not pass through a square that is attacked by an enemy piece.
The king does not end up in check. (True of any legal move.)

The King did not pass through an attacked square, only the rook.

Your bishop was on the diagonal, however the king does not move to b8, therefore it is a legal move.

FIDE Laws of Chess:
(2) Castling is prevented temporarily:
[a] if the square on which the king stands, or the square which it must
cross, or the square which it is to occupy, is attacked by one or more of
the opponent's pieces

Here the 3 squares are not attacked, so castling is allowed

Thanks guys! Must study how to castle

You must be Victor Korchnoi!

When your Bishop was still on c7, then indeed the black long castling wasn't allowed, because on c7 this Bishop attacks the d8-square, which the black King has to cross for a queenside castling. But after 17...Qe7 you played 18.Bg3 and left the d8-square uncontrolled, so that 18...0-0-0 was allowed, since only the black Rook a8 had to cross a square (b8) which is controlled by the Bg3. And Rooks are allowed to cross squares that are controlled by enemy pieces. Rooks may even be under attack before and after the castling.

#2 you copied from wikipedia cuz appears [1] of the references :V

lol

Sure did, wiki is the first source of information for almost everything =)