Iron Gates
Trajan, with the assistance of the Iazyges, led his legions into Dacia against King Decebalus, in the year 101. In order to cross the Danube with such a large army, Apollodorus of Damascus, the Romans' chief architect, created a bridge through the Iron Gates by cantilevering it from the sheer face of the Iron Gates. From this he created a great bridge with sixty piers that spanned the Danube. Trajan used this to strike deep within Dacia, forcing the king, Decebalus, to surrender and become a client king.