Castling Error

Although castling is usually a good move which keeps the king protected and connects the rooks, it can be an error. In a game of speed chess which I played at chess.com, my opponent's decision to castle was an error. My opponent was Sassang of Malaysia who played black. Here are the moves of the game along with my commentary:

1. d4 d5
2. c4 Nf6

Black's move is a surprise. More common moves are dxc, e6 and c6.

3. Nc3 dxc
4. e4 Bd7
5. Bxc4 e6

I regain the pawn.

6. Bg5 Bb4
7. e5 Bxc3+
8. bxc3 h6
9. Bxf6 gxf6
10. Nf3 f5
11. d5 Qe7

Black does not capture my pawns because with his king in the centre he wants to keep the position closed.

12. d6 cxd
13. exd Qf6

My d-pawn is isolated but not so easy to eliminate.

14. Qd3 Nc6
15. 0-0 a6
16. Rfe1 0-0-0

This move is bad because it allows a dangerous sacrifice. Black wants to connect his rooks, but his king has little protection on the queenside. Kingside castling is a better option, but even that is risky. Safer for black is Kf8 followed by Kg7 to connect the rooks.

17. Bxa6 bxa6

It is better for black to refuse the sacrifice.

18. Qxa6+ Kb8

This is the only legal move for black. The game is now lost.

19. Rab1+ Nb4
20. Rxb4+ Bb5
21. Rxb5#

Castling is usually a good move, but in this game it leads to black's downfall. With my advanced d-pawn, control of open files and better pawn structure, I take advantage of black's mistake. His decision to castle kingside convinces me to unleash the combination which begins with a bishop sacrifice.