Art Nouveau was an architectural and decorative art style that was popular from around 1890 to about 1910. In the USA it was often called "Tiffany style".
One of the best places in the world to see examples of this style is in Riga, Latvia. The building where the Art Nouveau Museum is located has a magnificent spiral staircase in this style.
1. Looking down from the top.
2. Looking up from the bottom