Chambord Castle
The original design is attributed, though with several doubts, to Domenico da Cortona, whose wooden model for the design survived long enough to be drawn by André Félibien in the 17th century. In the drawings of the model, the main staircase of the keep is shown with two straight, parallel flights of steps separated by a passage and is located in one of the arms of the cross. According to Jean-Guillaume, this Italian design was later replaced with the centrally located spiral staircase, which is similar to that at Blois, and a design more compatible with the French preference for spectacular grand staircases. However, "at the same time the result was also a triumph of the centralised layout—itself a wholly Italian element." In 1913 Marcel Reymond suggested that Leonardo da Vinci, a guest of Francis at Clos Lucé near Amboise, was responsible for the original design, which reflects Leonardo's plans for a château at Romorantin for the King's mother, and his interests in central planning and double-spiral staircases; the discussion has not yet concluded, although many scholars now agree that Leonardo was at least responsible for the design of the central staircase.
-Zoe (With the help of Wikipedia)