I visited Vienna in August 2015 and I spent 4 days exploring this magnificent city. I always knew Vienna was a beautiful city but I was pleasantly surprised at how well preserved it is and at how practically every facade, street and park is packed with historical, architectural and cultural sites to admire. The historic centre is huge and is much more difficult to cover when compared to Prague or Budapest. A day ticket for the tram costs 6 euros and it is worth buying to visit the farthest sites such as the Belvedere Palace (and Schonbrunn too). However, if you're fit and have enough time the best way to explore the city is on foot (paying attention not to end up with a stiff neck as you'll be looking up at the Jungendstil, Art Nouveau and Baroque facades and statues all the time!). The City Hall is where several important sites are situated: the Hofburg Palace, the Natural History Museum, the Fine Arts Museum, the Spanish Riding School, the Museum Quarter, Karlskirche, etc. Otto Wagner's creations are not only worth visiting but could be a WHS on their own as they really are of OUV. The Secession is my personal favourite in Vienna but all are really worth viewing. St. Stephen's Cathedral stands tall in the middle of the historic centre and I had long wanted to see its marvellous roof tiles. The Am Hof and nearby streets and buildings are always packed with people. Yet, they are outstanding to say the least. I believe Vienna is one of the best examples of historic centres in the world and my personal favourite after Rome and Paris. I'll definitely revisit in the near future (if only to allow for a day trip to Graz using the Semmering Railway).