3/25.- Chambord, the double spiral staircase
Leonardo da Vinci designed this very special staircase. The fact that it is double allows you to see someone else on the other staircase, but without ever meeting him. This allowed the king to go up and down without meeting his servants. Intelligent solution, isn't it?
Modern architects have used Leonardo's idea to allow the evacuation of double the people with only one staircase.