user15543: I wish to know the fastest possible castling checkmate, both queenside and kingside, that is vertical.
See my question about horizontal checkmates.

user11629: mate in seven for queenside castling:
[FEN ""]
1. e4 d5 2. d3 dxe4 3. Nc3 Kd7 4. Bh6 Kd6 5. dxe4+ Ke5 6. Qh5+ Kd4 7. O-O-O#

Might be worth checking whether it gets any faster if black is the one to castle.

user7297: Here is a mate (in 7) for kingside castling:
[FEN ""]

1. e4 f5 2. f3 fxe4 3. fxe4 Kf7 4. Qh5+ Kf6 5. Nh3 a6 6. Bc4 a5 7. O-O#


user15989: Mate in 6.0 for kingside.
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1 "]
1. e4 e6 2. Bc4 Bd6 3. Ke2 Qg5 4. Kf3 Nh6 5. Bxe6 fxe6 6. Qe2 O-O#

And 5.5 for queenside, even faster.
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1 "]
1. d4 Nc6 2. Nc3 Nxd4 3. Qxd4 e5 4. Qc4 Ke7 5. Bg5+ Kd6 6. O-O-O#

This should be fastest.

15543: <p>I wish to know the fastest possible castling checkmate, both queenside and kingside, that is vertical.</p>
<p>See my question about <a href="https://chess.stackexchange.com/questions/23847/">horizontal checkmates.</a></p>
 11629: <p>mate in seven for queenside castling:</p>

<pre><code>[FEN ""]
1. e4 d5 2. d3 dxe4 3. Nc3 Kd7 4. Bh6 Kd6 5. dxe4+ Ke5 6. Qh5+ Kd4 7. O-O-O#
</code></pre>

<p>Might be worth checking whether it gets any faster if black is the one to castle.</p>
 7297: <p>Here is a mate (in 7) for kingside castling:</p>
<pre><code>[FEN &quot;&quot;]

1. e4 f5 2. f3 fxe4 3. fxe4 Kf7 4. Qh5+ Kf6 5. Nh3 a6 6. Bc4 a5 7. O-O#
</code></pre>
 15989: <p>Mate in 6.0 for kingside.</p>

<pre><code>[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1 "]
1. e4 e6 2. Bc4 Bd6 3. Ke2 Qg5 4. Kf3 Nh6 5. Bxe6 fxe6 6. Qe2 O-O#
</code></pre>

<p>And 5.5 for queenside, even faster.</p>

<pre><code>[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1 "]
1. d4 Nc6 2. Nc3 Nxd4 3. Qxd4 e5 4. Qc4 Ke7 5. Bg5+ Kd6 6. O-O-O#
</code></pre>

<p>This should be fastest.</p>
