Ratiaria, originally conquered by the Dacians and later by the Romans, was known for its gold mine possibly exploited by Thracians. The Romans became involved around 75 BC, and by 29 BC, Ratiaria was a fortified city. Under Augustus, the Romans officially conquered the region, organizing it into the province of Moesia. Ratiaria became a key station with a legionary fortress and fleet. After the division of Moesia in 86 AD, it became the capital of Upper Moesia.
Legio IV Flavia Felix was stationed here until the conquest of Dacia (101-106 AD). Afterward, Ratiaria became a colonia named Colonia Ulpia Traiana Ratiaria (107 AD). In the 2nd and 3rd centuries, it prospered as a trade center. With the definitive abandonment of Dacia Traiana in 271, old castra were reopened. By 285, Ratiaria served as the capital of Dacia Ripensis, hosting the military governor and the Roman legion XIII Gemina. It later became an important Christian center in the 4th century, with recorded bishops and theologians. The city faced sackings by the Huns in 440-441 and the Avars in AD 586. Archaeological excavations started in 1958, revealing its historical significance.
Duino Castle was built by the Wallsee family in 1389 on the cliffs overlooking the Gulf of Trieste. It replaced an older castle from the 11th century. Over time, the Wallsee family disappeared and the castle, after having been used as a prison, became the residence of the Luogar and Hofer.
At the end of the 19th century it became the property of Prince Alexander von Thurn und Taxis from the Czech branch of the House of Thurn and Taxis. It remains with the family to this day with his great-grandson Prince Carlo Alessandro della Torre e Tasso, Duke of Castel Duino the current owner. The castle has been opened to the public as a museum and park.