Alexander the Great's route from the Aegean to the Danube, through Thrace, 335 BC
Also marked on the map is another possible route for the cavalry: from Philippoupolis via Serdica (Sofia) and Vratsa as far as Vidin on the Danube (instead of Russe). If, however, Alexander had wanted to avoid Philippoupolis and preferred to take the Triballoi by surprise, then the shortest route from the Aegean to the Danube would have been to follow the Strymon (Struma) basin up to the plateau of Sofia, travelling thence to Vidin via Vratsa. This natural passage, oriented northwards in a more or less vertical line, is the prehistoric metals route, as well as the route through which the horse entered the Balkans during the 3rd millennium BC, followed by invaders from the Carpathians and other Danubian regions with their eye set on the Aegean. Today it is known as the International Highway E79.