I was just looking at a game of mine where I have a knight in my opponent's castle. I was under the impression that once you move into your opponent's castle, you can never move out, but in this case BrainKing will let me move out.

I need to capture on this turn, and I can either capture one piece with a pawn, or two pieces by making a knight's chage out of the castle.

Is this a bug, or is it legal?

Game is here: Camelot (Ceiter vs. tenuki)

I'm not moving until I know the move is legit.