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

user15989: Kingside castling, 8.5 moves (fastest; no way for a king to reach the last rank any faster than this.)
[FEN ""]
1. e4 d6 2. b4 Kd7 3. Nh3 Kc6 4. Bb5+ Kxb5 5. Nc3+ Kxb4 6. Ba3+ Kxa3 7. Qh5 Kb2 8. Qb5+ Kxa1 9. O-O#

Queenside castling: 8.5 moves (again, fastest.)
[FEN ""]
1. d4 e6 2. g4 Ke7 3. Na3 Kf6 4. Bg5+ Kxg5 5. Nf3+ Kxg4 6. Bh3+ Kxh3 7. Qd3 Kg2 8. Qg6+ Kxh1 9. O-O-O#

There you have it. The theoretical fastest is 8.0 moves if you try to let black deliver mate instead of white, but I couldn't find a way to set up the mating net or to avoid locking the king out. Feel free to try.

user23224: I know this thread is old, but here is my improvement to kingside castling-8.0 moves. Black even has to do one useless waiting move (Ng4).
[FEN ""]
1. e3 e5 2. Ke2 c5 3. Kd3 Bd6 4. Ke4 Nh6 5. Kd5 Ng4 6. Kxd6 Qf6+ 7. Kc7 Qb6+ 8. Kxc8 O-O#

Queenside castling can be proven to be impossible in 8.0.

15543: <p>I wish to know the fastest possible castling checkmate, both queenside and kingside, that is horizontal.</p>
<p>See my question about <a href="http://chess.stackexchange.com/q/23843">vertical checkmates.</a>.</p>
 15989: <p>Kingside castling, 8.5 moves (fastest; no way for a king to reach the last rank any faster than this.)</p>

<pre><code>[FEN ""]
1. e4 d6 2. b4 Kd7 3. Nh3 Kc6 4. Bb5+ Kxb5 5. Nc3+ Kxb4 6. Ba3+ Kxa3 7. Qh5 Kb2 8. Qb5+ Kxa1 9. O-O#
</code></pre>

<p>Queenside castling: 8.5 moves (again, fastest.)</p>

<pre><code>[FEN ""]
1. d4 e6 2. g4 Ke7 3. Na3 Kf6 4. Bg5+ Kxg5 5. Nf3+ Kxg4 6. Bh3+ Kxh3 7. Qd3 Kg2 8. Qg6+ Kxh1 9. O-O-O#
</code></pre>

<p>There you have it. The theoretical fastest is 8.0 moves if you try to let black deliver mate instead of white, but I couldn't find a way to set up the mating net or to avoid locking the king out. Feel free to try.</p>
 23224: <p>I know this thread is old, but here is my improvement to kingside castling-8.0 moves. Black even has to do one useless waiting move (Ng4).</p>
<pre><code>[FEN &quot;&quot;]
1. e3 e5 2. Ke2 c5 3. Kd3 Bd6 4. Ke4 Nh6 5. Kd5 Ng4 6. Kxd6 Qf6+ 7. Kc7 Qb6+ 8. Kxc8 O-O#
</code></pre>
<p>Queenside castling can be proven to be impossible in 8.0.</p>
