model illustrating the staircase’s design

Wikipedia:

The royal Château de Chambord at Chambord, Loir-et-Cher, France, is one of the most recognizable châteaux in the world because of its very distinctive French Renaissance architecture which blends traditional French medieval forms with classical Renaissance structures. The building, which was never completed, was constructed by King Francis I of France.

Chambord is the largest château in the Loire Valley; it was built to serve as a hunting lodge for Francis I, who maintained his royal residences at the châteaux of Blois and Amboise. The original design of the Château de Chambord is attributed, though with some doubt, to Domenico da Cortona; Leonardo da Vinci may also have been involved. …

One of the architectural highlights is the spectacular open double spiral staircase that is the centerpiece of the château. The two spirals ascend the three floors without ever meeting, illuminated from above by a sort of light house at the highest point of the château. There are suggestions that Leonardo da Vinci may have designed the staircase, but this has not been confirmed. Writer John Evelyn said of the staircase “it is devised with four (sic) entries or ascents, which cross one another, so that though four persons meet, they never come in sight, but by small loopholes, till they land. It consists of 274 steps (as I remember), and is an extraordinary work, but of far greater expense than use or beauty.”