The marble-cased and metal-'studded' Post Office Savings Bank building is the Jugendstil (Art Nouveau) work of Otto Wagner, who oversaw its construction between 1904 and 1906, and again from 1910 to 1912. You can explore the back section of the building, where there's also a small museum with temporary exhibitions on design and a video section on the history of the building.
The Jugendstil design and choice of materials were innovative for the time, with the grey-marble facade held together by 17,000 metal nails, and an interior filled with sci-fi-like aluminium heating ducts and naked stanchions.