Dang, 21' to the first floor! That's twice as high as mine. I hope you have a landing or two in those stairs!!
Cross bracing is about all you can do, and that high up you shouldn't hit your head. I'd use timbers on the four corners.

Cross bracing is about all you can do, and that high up you shouldn't hit your head. I'd use timbers on the four corners.
LIVIN THE SALT LIFE 
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