Castling, is it necessary to take it? I've got a simple question, that I'm having difficulty finding an answer for...

If it is possible to castle (I know the conditions for it), and I decide to move the king to the square where it would end up in castling, do I have to "take the castling", i.e. move the rook too?
Yes. 

Otherwise this would be an illegal move because during castling the king moves two squares, but apart from castling it is only allowed to move one square.