Beneath two medieval towns - the craftmans' Gradec and the diocese called Kaptol - the 19th century brought the planned construction of the Downtown of Zagreb, with its rectangular grid of streets and residential houses, framed with long lines of parks in which nestled great public buildings (the National Theatre, Artists' Pavilion, the Academy of Arts and Sciences, the University Library, which is a splendid work of Art Nouveau, etc.). Due to its harmonious balance of architecture and green areas, Zagreb has become a perfect example of the "ideal city" of the 19th century, while many European cities were challenged by the chaos of transportation, industry and pollution in residential areas. On a hilltop above the city is the Mirogoj cemetery, with trees planted among the graves like in an English park, framed by neo-Renaissance arcades with domes of bronze (nowadays turned pale green with patina), forming a picturesque illusion of the city's "crown". At the end of the 19th and the beginning of the 20th century, along with Zagreb, Rijeka and Osijek have also developped well and steadily with a series of beautiful Art Nouveau houses.