If you have the Salzburg card take the number 25 from Rathaus. The bus is free and entrance is free.
The water gardens with the trick fountains are great, especially on a hot day, but the castle itself is also an interesting historical piece. An hour or so can be spent wandering the castle (more like a grand house) and reading about the history of the place and the people.
The gardens are big too. Nice for a stroll.