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I often hear that you should castle king-side first rather than queen-side.

What are the advantages of castling king-side as opposed to queen-side. Sometimes I like castling queen-side because when I do, my rook ends up on an open file.

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It's very difficult to answer without a specific position, but here are a few general thoughts:

Kingside

  • It is almost always faster to castle kingside because only two pieces need to be moved out of the way, and those two pieces have very natural squares (Nf3, Bb5/c4/e2).
  • The kingside pawns are usually left on their starting squares so the king is usually safer on the kingside than it would be on the queenside. This is in part because the a file can be weak after long castling.
  • In many positions, the move c3 (...c6 for black) is useful to support a d4 (...d5 for black) push. c3 is much safer after kingside castling. After queenside castling, c3 makes the b1-h7 diagonal weaker.
  • After short castling, it is possible to push the a and b pawns to try and gain space on the queenside without making weaknesses around the king. For example, the move a4 is frequently useful in the Ruy Lopez and the Slav.

Queenside

  • Usually white is the one to castle queenside because white gets to move first. White can use the initiative to develop faster and start an attack. Almost all opposite castling games feature white playing O-O-O and black playing ...O-O.
  • The biggest advantage of queenside castling is that the rook comes immediately to d1 (d8 for black). This means that queenside castling basically saves a move over kingside castling (no need for Rfe1).
  • Often white will need to play Kb1 to better protect the king (black can play ...Kb8). This is another tempo that must be "wasted" after castling long.
  • The king is often exposed on the c1-h6 diagonal because the d pawn is usually moved early in the opening. Kb1 mitigates this danger, but this is still something that must be considered.

Additional Thoughts

  • If you want an attacking game, castling to the opposite side of the board than your opponent is the easiest way to do this. The reason is that then the pawns in front of your opponent's king can be pushed forward to participate in an attack.
  • If you want a safe game, for example when your opponent has a lead in development, castle on the same side as your opponent. If your opponent tries to push the pawns on the same side as the kings, you will frequently be able to counter-attack.
  • When in doubt, kingside castling is almost never "wrong", but queenside castling is sometimes a grave error. In illustration, I present Boden's Mate:

Canal-NN final position

  • Good analysis: you however forget the "weakness" of the a2 pawn when castling queen side. That mate has little to do with castling though. (It looks more like bad development of minor pieces and not protecting the king's pawn structure). - It can just as well happen on the kingside. – paul23 Apr 19 at 14:50
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There are several reasons.

1) Castling kingside is faster, because there are only two pieces between king and rook, as opposed to three on the queenside.

2) When castling kingside, the king is placed to protect all the pawns on that side. Queenside castling leaves the rook pawn unprotected, and the king has to make another additional move (to b1 or b8) to protect it.

3) Castling on the kingside leaves the queenside freer for endgame purposes, like trying to obtain an "outside" passed pawn on the a file.

That said, there is one advantage of castling on the queenside in that it puts the rook on a center (d) file. If you've moved up your queen, you can reinforce it with the rook. If you've moved your queen off the file, and your opponent hasn't, the rook has the opposing queen in its sights. In short, you castle queen side for positional considerations (as opposed to tactical ones like the three above.

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