Here are some ideas for spiral staircases.
Some are borrowed from others, but I haven't noted
where I found them :-( (let me know if I can credit you).

BROWN
The brown is my favourite: it's sturdy, looks fine
and was fun to build.
It's combined from multiples Bracket 2 x 2 - 2 x 2 (3956)
on a 2x4 plate. The swivling point is just a round 1x1 plate.
Studs are covered as much as possible with tiles.
The plate 1x10 on the outside are mainly there for making
the whole as firm as possible.
The dark gray Plate, Modified 1 x 2 with 1 Stud (3794)
will eventually be replaced my brown ones,
so that the staircase will be completely brown.

GRAY
This one was also fun to build. I used almost all my 
Light Gray Plate, Round Corner 3 x 3 (30357) combined with
different shades of gray plates.
It definitely will find a place in one of my castle mocs.

Please let me know what you think.

Daan Bargerbos