castling short vs. castling long to surprise the opponent

Switching castling to other wing to surprise your opponent, at the last moment. Is it a good tool? How important could it be?

Please feel free to elaborate. Thank you for your valuable positive and innovative inputs. 

One strategical reason cold be pawn storm. Or simply the crowding of pieces on one side could be more heavier than the other side. 

i dont think anyone is gonna be suprise if you castle long or short

But how important could it be if one switches to the other side?

Very important! Good strategy if you can pull it off.

I like to be safe, so I go with short, but I've seen opponents crush me with castle long

That strategy works better in openings like caro kann or modern defense or London system. 

Is this true because of the pawn structure, or other reasons?

In those kind of openings, Centre is closed for some time , so you can keep the king in the centre and wait for sometime before you castle depending on which side seems safer.

Sometimes Black postpones castling in order to avoid runing into an attack... 

Sometimes White has the option of having a calm game or a wild game with opposite side castling. The decision where to castle to, however, should be determined by the positional features, not just the desire to suprise your opponent...

Also giving indications - by making positional move(s)  -of castling one-way but doing the other way. 

For beginners n Intermediate players, castling short is automatic for both colors. As one becoming a strong player, one castle long or stays uncastled depending on situation.

Let’s take a look at when it makes sense to castle opposite sides and when it doesn't.

You should castle on the opposite side when at least one of the following factors is true:

1. When you are up in development and your opponent has already castled, you should consider castling in the opposite side. That way you will have a clear game plan and will also be able to capitalize on your development advantage.
2. When you have a damaged pawn structure (doubled paws, missing pawns, far advanced pawns, etc.) on one of the sides you should consider castling on the other side.
3. When opponent’s pieces are especially active on one side of the board, it is usually best to castle on the opposite side.
4. If you want to complicate the game you may consider this option. That may be true if you must play for a win due to a tournament situation, when the draw is not enough. Also that maybe done when you're playing against a stronger opponent, who is much better in simple/technical positions. That maybe your best bet.

You should not castle on the opposite sides when at least one of the following factors in true:

1. When you are behind in development and you need extra time to develop your pieces, it is usually not a good idea to give your opponent a straight forward way of launching an attack.
2. When the opponent’s pawns are advanced towards the side you’re about to castle, it is not a good idea to castle there (especially if the opponent’s king is castled on the opposite side). It will just give him a positional edge in the attack.
3. When there are open/semi-open files in-front of the side you’re about to castle, you should probably reconsider your decision to castle there (especially if your opponent has castled on the other side). That will give him more attacking possibilities, such as rook lifts, various sacrifices, doubling of pieces on the file, etc.
4. If you playing against a weaker opponent you may want to avoid castling opposite sides, in order to avoid sharp game and keep everything under control.

Note: These are general rules, not laws, meaning that there are always exceptions to them. When you’re making a decision what side to castle you should always take your time and evaluate all “pros” and “cons” and base your decision upon your own analysis. This is a very important decision. It pretty much dictates which way the game will continue. Take your time and think twice.