What is the fastest horizontal checkmate for both castlings?

Question

I wish to know the fastest possible castling checkmate, both queenside and kingside, that is horizontal.

See my question about vertical checkmates..

Answer 1

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.

Answer 2

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.