text: 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 .
  
  ###
  
  Leonardo da Vinci was known for designing donjons such as these in Florence .