user23209: The opponent has the option of castling but can I still play a solid game without castling either side? I won some games in bullet time controls but only because position was too complex and opponent lost on time. Please answer with respect to different time controls. (my rating: ~1700 on lichess)

user7657: You want to know if castling is absolutely necessary.  It depends upon the position.  The main purposes of castling are to safeguard the king and connect the rooks.  In general, with an intact pawn structure the king is normally safer in the corner after castling where the opponent's attacking lines are fewer. In the center, it can be attacked from eiher side.  But if the position is blocked as it can become in a closed game, there may be no hurry to castle.  If a lot of pieces get exchanged off, it may then be desirable to have the king nearer the center for the endgame.  Which side your opponent has castled on can also be a factor, since you don't want to castle into a potential pawn storm on opposite side castling.  So there's really no hard and fast answer to your question, and it depends largely upon the circumstances.  I might add that I play a lot of games against the computer when it doesn't castle, and I find it much easier to attack the king then, with an impressive record of resulting wins.

user2001: There are openings where you can keep your King on its initial square for more moves. I personally prefer to castle relatively quickly. Castling has a good psychological advantage of taking care of King safety before planning the next phase of the game.
A game in my mind goes something like this.

initial development to control the center and prepare castling
castle the King
continue taking control of the center
launch an attack or counter-attack

If I keep my King in its initial square, I have to spend extra time on considering its safety. With a castled King, I have clear ways of defending it from an attack, because I know the usual patterns of attack and defense depending on which side I castled. If my opponent waits with castling, I start disrupting their structure on the side where I do not want them to castle.
One situation where I delay castling is to see where my opponent castles, in case I want to either avoid or ensure a castling on opposite sides. Positions where two players castled on opposite sides are very sharp and have a specific character. Pawn storms will be launched on both sides and the position will get complicated, dynamic and require a lot of calculation.
Those are some of my thoughts on the topic. I hope it helps you to decide what to do with your King. I think you can use the same principles across all time controls, just as Brian pointed out. I hope my answer added something useful. Let me know if you have any comments.

user4418: The purpose of castling is to make the king safe. How important castling is in any particular game depends almost entirely on how safe the king is in the center. If the center is completely open (say all four central pawns have been exchanged) and many pieces are still on the board then it is very dangerous to leave the king in the center. If the center is completely closed, however, then the king may be much safer in the center.
On the other hand there are openings (Berlin Defence to the Ruy Lopez for example) where the queens come off very quickly and one side having an uncastled king is fine and confers no disadvantage.
Time controls have nothing to do with this. Having an unsafe king while your opponent's king is safe just makes your opponent's game much easier regardless of time control. You could even argue that the shorter the time control the less time you have to make plans so the easier it is for the player with the safe king because all they have to do is attack the opponent's unsafe king.

23209: <p>The opponent has the option of castling but can I still play a solid game without castling either side? I won some games in bullet time controls but only because position was too complex and opponent lost on time. Please answer with respect to different time controls. (my rating: ~1700 on lichess)</p>
 7657: <p>You want to know if castling is absolutely necessary.  It depends upon the position.  The main purposes of castling are to safeguard the king and connect the rooks.  In general, with an intact pawn structure the king is normally safer in the corner after castling where the opponent's attacking lines are fewer. In the center, it can be attacked from eiher side.  But if the position is blocked as it can become in a closed game, there may be no hurry to castle.  If a lot of pieces get exchanged off, it may then be desirable to have the king nearer the center for the endgame.  Which side your opponent has castled on can also be a factor, since you don't want to castle into a potential pawn storm on opposite side castling.  So there's really no hard and fast answer to your question, and it depends largely upon the circumstances.  I might add that I play a lot of games against the computer when it doesn't castle, and I find it much easier to attack the king then, with an impressive record of resulting wins.</p>
 2001: <p>There are openings where you can keep your King on its initial square for more moves. I personally prefer to castle relatively quickly. Castling has a good psychological advantage of taking care of King safety before planning the next phase of the game.</p>
<p>A game in my mind goes something like this.</p>
<ol>
<li>initial development to control the center and prepare castling</li>
<li>castle the King</li>
<li>continue taking control of the center</li>
<li>launch an attack or counter-attack</li>
</ol>
<p>If I keep my King in its initial square, I have to spend extra time on considering its safety. With a castled King, I have clear ways of defending it from an attack, because I know the usual patterns of attack and defense depending on which side I castled. If my opponent waits with castling, I start disrupting their structure on the side where I do not want them to castle.</p>
<p>One situation where I delay castling is to see where my opponent castles, in case I want to either avoid or ensure a castling on opposite sides. Positions where two players castled on opposite sides are very sharp and have a specific character. Pawn storms will be launched on both sides and the position will get complicated, dynamic and require a lot of calculation.</p>
<p>Those are some of my thoughts on the topic. I hope it helps you to decide what to do with your King. I think you can use the same principles across all time controls, just as Brian pointed out. I hope my answer added something useful. Let me know if you have any comments.</p>
 4418: <p>The purpose of castling is to make the king safe. How important castling is in any particular game depends almost entirely on how safe the king is in the center. If the center is completely open (say all four central pawns have been exchanged) and many pieces are still on the board then it is very dangerous to leave the king in the center. If the center is completely closed, however, then the king may be much safer in the center.</p>
<p>On the other hand there are openings (Berlin Defence to the Ruy Lopez for example) where the queens come off very quickly and one side having an uncastled king is fine and confers no disadvantage.</p>
<p>Time controls have nothing to do with this. Having an unsafe king while your opponent's king is safe just makes your opponent's game much easier regardless of time control. You could even argue that the shorter the time control the less time you have to make plans so the easier it is for the player with the safe king because all they have to do is attack the opponent's unsafe king.</p>
