The historic center of Riga , Latvia , with `` the finest collection of art nouveau buildings in Europe '' , was inscribed on the list in 1997 in part because of the `` quality and the quantity of its Art Nouveau\/Jugendstil architecture '' . Four Brussels town houses by Victor Horta were included in 2000 as `` works of human creative genius '' that are `` outstanding examples of Art Nouveau architecture brilliantly illustrating the transition from the 19th to the 20th century in art , thought , and society '' . The historic center of Riga , Latvia , with `` the finest collection of art nouveau buildings in Europe '' , was inscribed on the list in 1997 in part because of the `` quality and the quantity of its Art Nouveau\/Jugendstil architecture '' , and four Brussels town houses by Victor Horta were included in 2000 as `` works of human creative genius '' that are `` outstanding examples of Art Nouveau architecture brilliantly illustrating the transition from the 19th to the 20th century in art , thought , and society '' .