Castling is is when the King and the Rook sort of swap places. If your White castling "o-o" is when the King moves to G1 and the Rook goes to F1. Castling "o-o-o" is when the King moves to C1 and the Rook goes to D1. When your Black castling "o-o" is when the King moves to G8 and the Rook goes to F8. Castling "o-o-o" is when the King moves to C8 and the Rook goes to D8. There are rules to be met when castling and there are at least 4 rules for castling on both sides. Neither the King or the Rook intended to be castled moved prior to castling. No pieces are in the way or in between the King and the Rook before castling. No castling while your in check. No castling through check. These are the rules. Enough of the rules let's talk about the upside and downside of castling. The upside is if there is one of your opponent's pieces about to put your King in check or is in your side of the board threatening your other pieces castle and your King is safe with a barrier of the other pieces gaurding it. It may just save you from a near checkmate or just to backup some piece in need of assistence. It creates a wall your opponent does not want to place their pieces. The downside in castling is your King is trapped in a packed space with no where to go. It is especially susceptible when your opponent lines up his/her pieces to a weak spot in your defense and you are helpless when a Queen comes in backed up by either a Rook, Knight, or Bishop. Your are forced to resign because your nearly checkmated. Choose with thought and plan ahead to stop your opponent from getting in. If you do all of these your opponent is going to be trying for hours with no avail to get in. Castling is a tool use it like a tool and master it. Time it with great accuracy.