User blog comment:B214/Question and Answers 6/@comment-24098614-20161024211044/@comment-29963364-20161025055721

In real Chess, you can only perform castling if your King and either of your Rooks haven't moved yet. So yes, it's a one-game-move only.

As for the Rating Game in the DxD series, it might be a move you can use as often as you want. Although I'm sure the Rating Game Committee will have rules and regulations on that depending on the type of game it is and who the players are.