Ultimate En Passant Castling Chess

The idea of this variant is to allow passing squares to be used to chain together multiple moves, as well as capture any piece by en passant.

The board is a standard 8x8 and all the normal pieces are used.

Any square that a piece passes through may be castled to by any friendly piece immediately following that move on the same turn, including the starting square, using its normal move. The piece being castled with moves first, and does not block its own starting square. Despite moving first, it also doesn't block its destination square for pieces castling with it, but does block its destination square for castling with other pieces.

A piece cannot castle with itself, or castle with the same piece twice in one turn, but it is legal to castle a piece twice in one turn with different pieces.

When passing through a square, any piece may be captured en passant. A piece which castles with another piece may be captured en passant on its starting square, but the first piece being moved in a turn may not, regardless of whether any other pieces castle with it.

The king has a specisl move: it may castle with a piece which has moved by moving 2 squares in any orthogonal or any diagonal direction, passing through the intervening squares. It cannot jump over pieces it is not castling with in doing so. The king cannot expose itself to en Passant capture, so Castling out of check only works if the castling blocks the starting square or captures the checking piece, and the destination and intermediate square are also not exposed to capture.

There is no rule against castling with many pieces to one, or castling with a castled piece. However, you must complete all of the castling with a move before castling with a move that castled with it.

Every square passed through by an enemy piece last turn and the starting squares of castling pieces are en passant squares for those pieces. There is no limit to how many pieces can be captured normally or en passant on the same square or in the same turn.

Non-capturing moves to an en Passant square do not capture whatever would be captured en passant by a capturing move there, but are perfectly legal moves.

When a pawn promotes, it retains its identity for purposes of castling and en Passant until the end of the opponent's turn. That is, castling a pawn into promotion still prevents the resulting promoted piece from then castling with the same piece again, and doesn't prevent it from being captured en passant on the opponent's turn.

Here's a GIF of a game I played against myself. There's some questionable tactics involved so don't take this as high quality gameplay. I was deliberately doing some "very aggressive" stuff as black and got punished hard.

Also I realized I miss the opportunity to capture White's rook by en passant on its starting square at one point.

Reminds me of Grolman's Chess. Looks interesting.

You can get Grolman-like move sequences, but they are not forced in any particular order, and of course you need to think about whether they expose you to en passant captures.

Nice!