The huge Hofburg is the most important secular building in Vienna, once the center of the powerful Habsburg Empire. The old Hofburg, with its many different sections and courtyards, was built and renovated many times between the 13th and the early 19th Centuries. The Neue Burg (New Palace) was developed in the late 19th and early 20th Centuries. Today, the Hofburg is home to various museums such as Völkerkundemuseum, Schatzkammer, Nationalbibliothek, Albertina, Theatermuseum, Prunksaal, Spanische Hofreitschule and Kaiserappartements and the library, as well as the offices of the Austrian President. At the chapel, the choir of the Vienna State Opera performs mass. The Imperial Treasury houses the Emperor's Crown of the Holy Roman Empire and the Austrian Emperor's Crown. The furniture and the other collections spell class and luxury. In January, the glamorous Wiener Ärzteball takes place here.