Nuremberg is the largest city in Franconia and the natural centre of North Bavaria. In terms of population it is the second largest city in Bavaria and the thirteenth largest in Germany. Together with its neighbours Fürth, Erlangen, Schwabach and the surrounding rural districts, Nuremberg is part of one of Europe’s most important economic areas, where the EU and its Eastern neighbours meet. Nuremberg is surrounded by attractive landscapes: to the East the Franconian hills, to the North "Franconian Switzerland" and to the South-West the Franconian Lakes. Centuries of art and architecture made Nuremberg a treasure. During the 15th and 16th centuries, Nuremberg enjoyed a cultural flowering that made it the center of the German Renaissance, bringing together Italian Renaissance and German Gothic traditions. In the artists' workshops were found such great talents as Veit Stoss, Peter Vischer, Adam Krafft, Michael Wolgemut, and above all, Albrecht Dürer. Koberger set up his printing press here, and Regiomontanus built an astronomical observatory. Here, too, flourished the guilds of the Meistersingers, composed of prosperous artisans; Wagner made their most famous member, Hans Sachs, the hero of his opera Die Meistersinger von Nuremberg. Many of Nuremberg's most important buildings, including some of the finest churches in Germany, have been restored or reconstructed. The old part of the city, the Altstadt, lies mainly within a pedestrian zone. Today's visitors can see the ruins of the ramparts that once surrounded the city as well as more modern sites, such as the Justice Palace, where the War Crimes Tribunal sat in 1946. Visitors can also see the Zeppelinfeld arena, the huge amphitheater where, from 1927 to 1935, Hitler staged those dramatic Nazi rallies that were immortalized by Leni Riefenstahl in Triumph des Willens (Triumph of the Will). Hitler's architect, Albert Speer, constructed what has been called a "concrete mecca," whose grounds today have been turned into a park with apartment blocks, a trade fair, and a concert hall. Speer's Congress Hall, larger than the Colosseum in Rome, has become a recording studio and warehouse.