I am relatively new to chess. I learned how to play as a kid, but now as an adult I am taking it a little more seriously. I am a teacher at a middle school and some kids asked to start a chess club. So now I will play with students a couple times a week. The more I play against some of the better students I find myself working my way into checkmate after castling. This happens mostly once my opponent brings their queen to a4 or h4 when I play as white. So my question is this:
Why is castling such good protection?
During a "normal" game of chess, the players move the center pawns and reduce the protection they provide the king. (AlphaZero often brings the rooks out by playing h4 and Rh3.) To avoid the classical "catching the king in the center" attack, the player castle, normally on the kingside. This replaces the pawn coverage and makes it harder to attack the king. On the side of the board, it's harder to organize the pieces. The defending pieces are better placed for an adequate defense. When the king is on the kingside, the pawns prevent the opponent's pieces from approaching, the knight on f3 protects h2, and the rook protects f2. However, this also leads to back rank mate attacks due to the now limited king movement.
Welcome to lichess!
It is a great thing that you are running a chess club, and I hope you keep playing and enjoying chess and that your students do as well.
However, I am confused by your question, there are several points that are rather obscure.
Can you please make it more clear whether:
(1) Is it you or your opponent who are getting checkmated?
(2) Are you castled here or not? If so, kingside or queenside?
(3) How is the black queen getting to a4? And how does it pose a threat to the white king from there?
(4) Are you asking why you are not getting checkmated as much when you do castle? Or why is the king so vulnerable when it is not castled? Or why does the king get checkmated anyway if it is castled?
It guards your king from checks and connects your rooks. Castling doesn't mean that you can't get checkmated. You need to be attentive and avoid getting checkmated. Usually there is at least a knight to defend the pawns and squares on the kingside. Sometimes you might have to move a pawn to make checkmate impossible. In the meantime try to attack the opponents king or pieces.
If you play a game online, you can post it in the forum "Game analysis" where people can tell you how you could have avoided the checkmate.
The short answer to your question is your knight needed to be on f3, to stop the queen coming to h4 or g5. When the knight is not on f3, then the queen can go to h4, knight comes to g4, and sometimes unstoppable mate to follow.
The REAL QUESTION seems to be, "if castling is supposed to be safe, why am i still getting checkmated?"
The medium length answer is DEFENSE. If 2 or more of your opponents pieces are aimed at your kingside, or are able to get there in the next few moves then you need to be aware and calculate your defensive resources and decide if you need to take prophylactic measures. Also, sometimes castling itself is actually a blunder, so before immediately castling, make sure that you aren't walking into a mate in 1 trap, where your opponent can threaten mate immediately or even in a couple of moves. Be sure that it is safe to castle before castling. The point is to get out of danger, so you don't want to walk into it.
I have listed a few of your main defensive resources in a normal kingside castle position. These apply to all openings as they address thematic checkmating ideas that beginners often know about.
1. "Knight on f3" as white, consider your knight on f3. This knight is actually a powerful kingside defender. As a defender, the knight controls the h4, and g5 squares where your opponent would like to place his queen for an easy checkmate. Also the knight protects the vulnerable h2 square, where the opponent's queen sometimes wants to deliver checkmate. If your knight is not on f3, then your opponent can put his queen on h4, knight on g4, and this type of attack can be dangerous. Of course you can't always have your knight stuck on f3, but make sure one of your knights is able to come back to f3 if the situation calls for it, especially in the opening not having the knight on f3 allows for some quick checkmates for black.
2. "h3" Another defensive prophylactic resource is moving the pawn to h3 ahead of time. Sometimes playing h3 ahead of time will limit your opponent's knight, etc. from accessing the juicy g4 square, and will make the h2 square less of a target for black's pieces. However, other times, h3 itself becomes a target for a bishop, knight, or rook sacrifice, where you king will end up wide open and exposed. Be careful if your opponent has 2 or more pieces aimed at your h3 pawn, because they likely have the ability to sacrifice and gain a crushing attack.
3. "g3" Like the h2 pawn, the g2 pawn (the one in front of white's king when castled kingside) is generally weak because it is only protected by the king. It can be a clearcut target for your opponent so you need to be aware of your defensive options. First of all, the checkmate you want to avoid is your opponent getting his queen on the g file, and adding a second attacker by putting his bishop on h3, or his knight on f5 for example. Playing pawn to g3 can be a useful resource to eliminate the weakness of the g2 pawn, making it a strong g3 pawn. However, make sure your rook won't be under attack (this means playing rook to e1 before your opponent makes threats) The other downside is that it weakens the dark squares all around your king. The last thing you want is your opponent getting his queen onto h3 and his bishop or knight onto f3 or visa versa. so unless you have fianchettoed the bishop to g2, this will be a weakness for black to exploit. Instead of playing g3, you can also block the g file by putting a knight in front of your pawn, but this is usually a temporary solution unless you can chase the queen away quickly.
4. Knight on d1 or bishop on c1. These are secondary defensive resources which won't be necessary all of the time. These pieces protect the g2 square in front of your king.
5. Preparing for a king walk. Sometimes your opponent is going to sacrifice material for an attack and there's nothing you can do about it. This doesn't always mean checkmate. In some cases, you just need to accept the sacrifice, and make room for your king to escape from the kingside. This might mean moving your rook, or sometimes the f pawn. When the dust is settled, you may just come out a piece ahead!
6. "Use your Queen" Your queen as a defensive piece. Your queen is a very flexible piece. While we usually think of the queen as a powerful attacker, never forget she is a strong defender. If you sense danger, make sure she can come back close to your king to help out with the defense. Once she is closeby, you can try to force a queen trade. If your opponent doesn't want to trade queens, they will probably have to retreat with their queen, thus stifling the attack. If you can get the queens off of the board, then your checkmating concerns are reduced and you can focus on positional play.
7. " Trade other pieces" Although its not ideal to enter a drawish position, the opponent on the defense will often benefit from trading pieces. The more pieces you can trade off the board, the less threats your opponent can make and you can enjoy a more positional end game.
8. "Beware the pawn storm" If you have just castled, and your opponent instantly plays h5, then sometimes you will need to respond with h4, or h3 depending on the circumstance. If your opponent continues storming with his pawns towards your king, then try to make a counterattack happen faster, and always leave an escape route for your king.
You don't always have to castle so you can play some games without castling and see what happens. In many cases it's not the best strategy but you will be able to learn why castling is a good defense or, actually the king in the center itself can be a strong resource sometimes especially when queens are traded in the opening.
@rhelmstedter Castling (in most scenarios) is a logical consequence of the fundamental principals both sides should play by during the opening.
In the opening, both players want to:
Principle 1. Control the central squares
Why? The player with control of the center will have a natural initiative due to his advantage in space, which facilitates the maneuvering of his pieces and hinders the maneuvering of his opponent's pieces.
Principle 2. Develop the pieces
Why? Both players should quickly develop their pieces towards the center in order to devote as many of their resources as possible towards central control.
Principle 3. Get the king to safety
This follows from the first and second principles – if both sides are following the above two principles with every move, the center of the board will soon turn into a war zone filled with pieces of both colors. If either player leaves his king in the center, his opponent should rip open the center (i.e. open the central files) and use his centralized forces to launch a forceful attack on the unfortunate king.
Except in certain scenarios (such as a closed center), the action of castling follows all three of these principles.
Satisfaction of Principles 1 and 2. Castling brings the rook from an inactive corner to a square close to the middle of the board (f1/d1 for White, f8/d8 for Black) from which it can better participate in the fight for the center. Additionally, castling removes a major obstacle (the king) in the way of connecting the rooks, meaning that castling improves piece coordination. Finally, even if the f or d-files are blocked or are not the best squares for the rooks, since castling moved the king away from the center, the rooks can now move to active files (e.g. the e-file) that were previously inaccessible due to the position of the king.
Satisfaction of Principle 3. Castling tucks the king behind a (hopefully intact) wall of pawns away from the center. After castling, since the king will not be a sitting duck in the center, the player with the castled king can play more dynamically and explosively in the center (as he no longer has any fear of getting his own king checkmated by his actions).
Hope this helps!
Concrete games would be useful. Play again with this player and write down the moves. Upload them here. That will make it easier for us to give good answers.
The others gave more or less complete answers to your general question why castling is good. So i will instead post some example games, two of them given by Kasparov in his book 'Kasparov teaches chess' (i cant believe that Lichess does not have them in its database):
The first has been played by Morphy. He catches the king in the center:
Now a more modern example, played by Garri, which had made big impressions to me when i once replayed it:
But such accidents can also happen when a player has castled and some of the defending pawns are missing. Here is another classical example played by Lasker (not Emanuel, but Edward). Here we also have a Queen playing the decisive role: