Each individual cedar tread is the full thickness of a riser; the natural curvature of the slab edges provides the code-required nosing. A custom iron railing, not yet installed, will protect the adjacent windows.
Because they've evolved to bend in the wind, large trees are surprisingly flexible.
To eliminate any distortion of the trunk during layout and initial construction of this spiral stair, the builders carefully braced the center column in the position it would later assume when vertical.