I was playing v the cpu and i did not notice that castled, only that it came up with a pop up saying this was a legal move.
I googled it and it gave me the mechanics on how its done and the parameters to make it legal. But what i did not see was a tactical or strategic reason to do so.
So , why would you castle
To gain a tactical advantage where the King is better protected. It’s a strategic advantage if you can concentrate on the attack and worry less about defense.
http://www.learnchessrules.com/castling.htm
Add to that that protecting the king is important, moving it closer to the location where the rook was means that the king will be the one behind the wall of pawns after one castles.
The benefit of castling is twofold. At the most obvious level, castling brings the King away from the center of board, and it brings a Rook closer to the center of the board.
The King (generally) wants to move away from the center in order to diminish tactical opportunities for the opponent that would involve your King: pins, skewers, forks, etc. In the early and middle game, the Rooks want to be placed either on open files or support central pawns.
The underlying concept that makes both of the above statements important is the value of the central squares. Knights, Bishops, and Queens become more powerful as they approach the center of the board because they have more moves available to them. For example, a knight sitting on a1 only has two (potentially) legal moves available, whereas a knight sitting on d5 has eight. This is why much of the game involves trying to control the center.
I rarely castle, but when I do it is for one of two reasons:
It can bring the rook out faster and aid in pressing an attack.
If I find myself in an “oh, shit!” position (but not in check - you can’t castle in check), castling can frequently move the king into a more defensible position. Sometimes you will back yourself into a corner doing that, though.
But I am far from a chess master, so hopefully somebody else will come along and give a more detailed analysis.
It’s an excellent move both defense and offense; protect King and launch Rook into battle. The sooner in the match you can castle the better. Although I would like to hear the experts advice.
Going to move this to the Game Room.
It also links your rooks together so they can better protect eachother (once all the other pieces are out of the back row)
A positional game gets all your pieces (read: Not Pawns) into play as early as practical. Have your Bishops control the long diagonals, get your Knights out from the first rank, and have your Rooks aligned on the center files.
From this position, you can mount a formidable defense against almost any assault your opponent may launch, and you can, in turn, launch a counter-strike in a flash. Key to this advantage is having your Rooks out of their confining corner positions. Hence, the tactical strategy of Castling.
Cuts both ways though. I remember as a kid playing my grandad and always always castling as soon as possible. Can’t tell you the number of times he mated me simply because my king was trapped behind that stupid line of stupid pawns.
Related question: when is Queen’s-side castling to be preferred?
As posters have said, castling is good because:
As a professional, I teach beginners to always try to castle.
N.B. Strong players will happily sacrifice a pawn or two to keep the enemy King from castling.
Not a good idea 
Your advice is generally sound*, but chess is **far more difficult **than you suggest! 
(*Inexperienced players should always use a couple of pawns to control the centre; bishops are just as good on other diagonals)
Back rank mates are a pain. But it’s much worse not castling…
King-side castling is easier to arrange. Queenside castling often leads to exciting positions where the players have castled on opposite sides and there is a race to attack each other’s King.
Here’s a sample line:
White could advance in the centre, but there could instead be fierce attacks against both Kings (traditionally you throw your pawns forward to open lines.)
Here is a thread with a training game and lots of advice:
You see it often in the Sicilian Dragon.
King side castling has a potential disadvantage in costing tempi. A queen side castle will get that rook to a more central location.
Also, depending on your opponent’s line of attack, there may be too much of a focus on one side of the board. So, an appropriately timed queen side castle can severely disrupt strategy and get pieces out of position.
Not only that, but you can’t castle if any of the squares between the king and rook are threatened.
However, you can castle to get a rook out of being attacked.
But the tradeoff is that a king side castle usually provides better protection for the King. So it really comes down to personal preference/board situation.
As I understand it, the main determiner of which way you’ll castle in any given game is just which side it’s easier on. Ultimately, you’ll want to get all of those pieces out onto the battlefield, but depending on the situation, some developments will be stronger than others, so whichever side opens up first, that’s the side you castle on.
And I tend to think of castling as more an offensive move than defensive. The rooks are quite powerful once they’re developed, but an uncastled rook is a lot harder to develop than a bishop, knight, or queen: You have to either let the pawn in front of it capture something or be captured, or do a time-wasting dogleg around it.
I think that might be the key. My father was a much, much better player than I ever would be. I always castled as a defensive move. How I used the rook was something of an afterthought. My father castled offensively. He had a plan for his rook, usually on the very next move.
You have no idea. I’ve played games where I’ve committed so many strategic and tactical errors that I think my dog could have beaten me. I love the game, but I have to admit, I pretty much suck at it.
It really depends on the game.
Of course, it’s a little of both, but there are many examples where it’s a defensive move to establish eventual control.
If you look at many Indian defenses for Black, you cede control of the center and usually end up castling king side. From this position, you can chip away at White’s control and/or wait for White to overextend.
Very informative, thanks all
Declan
Correction: if any of the squares between the current position of the king and the position of the king after castling are threatened.