Confused yet? Well, don't worry, there's only one more part to this rule. You can't castle if either of the two squares your King has to move through is attacked by your opponent.
You can see here, that White cannot Castle because the Black Knight is attacking the first square that the King has to move through.
Why?What is the purpose of Castling? The reason you castle is to help get your King to a safer place on the board, and also to help get your Rook to a place where is can get out from behind the pawns.
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