At Some point during the opening of the chess game most players will likely attempt to castle. Castling provides the player with a good line of defense for their king. Typically the player who has castled has at least three pawns in front of their king, a rook on the open side of the king, and one space followed by the edge of the board on the other side of the king. Most of the time it is a good idea to castle because of the protection it offers the king.
When playing against someone who has castled their king, which will frequently happen, it is a good idea to begin a strategy that allows you to attack your opponents king side. In other words, attacking those pawns that are protecting your king. If you are able to, you want to slowly but surely place your pieces on the opponent's king side so that you can overwhelm the pieces that are protecting your opponent's king. Depending on the situation it may even be worthwhile to sacrifice a piece in order to disassemble your opponents pawns that are protecting the king. You will have to decide if it is prudent to sacrifice a piece or not when attacking the opponents king side defense.
Being able to dismantle your opponents king side defenses, while at the same time maintaining your own kings side defenses, will give you a huge advantage in the game. Chess players who are able to destroy the opponents king side defenses and keep their own defenses in tact usually win the game. Use this as a strategy when you feel that you are perhaps a piece or two ahead, or when you have tempo going in your favor.