Postsparkasse (Austrian Postal Savings Bank), Vienna

The Postsparkasse building in Vienna, built by Otto Wagner in 1903, revolutionized architecture in Vienna. An early example of a building done with a steel grid system, and then reinforced with concrete, this type of construction is now done the world over, and is considered pretty much standard fair; however, in the early 1900s, the secessionist design and choice of materials were – and still are – considered a marvel to behold. You need only compare it with the nearby structures, especially the pompous ministry building at the other end of the park, to feel what a revolution this must have been.

The building's clear lines and cool elegance, achieved by the usage of steel, concrete and glass, give it a solid and impenetrable look. The skylight in the main hall nevertheless allows for natural light to reach the interior of the building, making it light and airy at the same time.

On weekdays, you can explore the back section of the building which houses a museum (entrance is free) dedicated to Otto Wagner and his building, along with various temporary exhibitions on design. There's a good documentary (in German only) about Wagner's life and work, detailing how he designed everything in the Postsparkasse – even down to the chairs and tables used. And it's all there to see, along with plans. Well worth a visit.