Move the king to his final position, e.g. kingside slide him from e1 to g1. You can try it out on the site's analysis board if you want: at the top menu, click under "More.../Analysis Board & Editor".
Castling..
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You only have to make the king move, the rook will move itself.
Just remember that you can't castle into check, out of check, nor can the king pass over a square which is under surveillance of an opponent's piece. The king and the rook with which you castle must never have been moved during the game (therefore you can only castle once per game). The squares between the king and the rook with which you castle must all be empty.
Does anyone know how to castle on here? I thought you could not until I played a game against the computer and the opponent castled.. Any suggestions? Live play as well? Thanks!