premise: It is believed that Leonardo da Vinci, whom Francois brought to the Loire Valley in 1516, participated in the design, especially of the central four-towered donjon ' which makes a dream palace out of a classically feudal castle keep ' and the celebrated double-ramped spiral staircase in the donjon's center, which enables people to go up and down without meeting.
hypothesis: Da Vinci was brought to the Loire Valley.