Throughout history the valley of the River Danube has been a significant thoroughfare across eastern and central Europe, but the river was also for centuries a major frontier of the Roman Empire. Flowing from Black Forest to Black Sea, this international waterway today passes through several major cities, including Vienna, the beautiful Austrian capital known as the City of Music and also the City of Dreams. Vienna has roots in a Celtic past and a history as a city that spans two millennia. Vienna has guarded the gateway to Europe as a fortress, and as an imperial capital has made many contributions to the world's cultural heritage.