The Danube is the second longest river in Europe, flowing through most of Central and South-East Europe, and the largest cities on the river are Vienna, Budapest, Belgrade and Bratislava, all capitals of their respective countries. The Danube Valley also has six capital cities: Bucharest, Sofia, Zagreb, Ljubljana, Sarajevo and Pristina, the capital of Hungary Budapest is probably the most beautiful city on the Danube, with several famous classical buildings along the Danube. Includes the Hungarian Parliament and Buda Castle.