Regensburg is a city in Bavaria, Germany, located at the confluence of the Danube and Regen rivers, at one of the northernmost points of the Danube. To the east lies the Bavarian Forest. The large medieval centre of the city is a UNESCO World Heritage site, and the first settlements in Regensburg date to the Stone Age. The Celtic name Radasbona was the oldest name given to a settlement near the present city and around AD 90, the Romans built a fort there.
In 179, the Roman fort Castra Regina was built for Legio III Italica during the reign of Emperor Marcus Aurelius. It was an important camp on the most northern point of the Danube: it corresponds to what is today the core of Regensburg's Altstadt (“Old City”) east of the Obere and Untere Bachgasse and West of the Schwanenplatz.
Between 1135 and 1146, the Stone Bridge across the Danube was built at Regensburg. This bridge opened major international trade routes between northern Europe and Venice, and this began Regensburg's golden age as a residence of wealthy trading families. Regensburg became the cultural centre of southern Germany and was celebrated for its gold work. In 1245 Regensburg became a Free Imperial City and was a trade centre before the shifting of trade routes in the late Middle Ages.
At the end of the 15th century in 1486, Regensburg became part of the Duchy of Bavaria, but its independence was restored by the Holy Roman Empire ten years later. The city adopted the Protestant restoration in 1542 and its Town Council remained entirely Lutheran. From 1663 to 1806, the city was the permanent seat of the Imperial Diet of the Holy Roman Empire, which became known as the Perpetual Diet of Regensburg. Thus, Regensburg was one of the central towns of the Empire, attracting visitors in large numbers