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. Around AD 90 the Romans built a small "cohort-fort" in what would now be the suburbs.
In 179 the Roman fort Castra Regina ("fortress by the river Regen") 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. It is believed that even in late Roman times it was the seat of a bishop, and St Boniface re-established the Bishopric of Regensburg in 739.
From the early 6th century, Regensburg was the seat of the Agilolfing ruling family. From about 530 to the first half of the 13th century, it was the capital of Bavaria. Regensburg remained an important city during the reign of Charlemagne. In 792, Regensburg hosted the ecclesiatical section of Charlemagne's General Assembly. The bishops in council condemned the heresy of Adoptionism taught by the Spanish bishops, Elipandus of Toledo and Felix of Urgel. After the partition of the Carolingian Empire, the city became the seat of the Eastern Frankish ruler, Louis II the German in 843. About two years later, in 845, fourteen Bohemian princes came to Regensburg to receive baptism there. This was the starting point of Christianization of the Czech people, and the diocese of Regensburg became the mother diocese of Prague. These events had a wide impact on the cultural history of the Czech lands, as consequently they were incorporated in the Roman Catholic and not into the Slavic-Orthodox world. The fact is well remembered, and a memorial plate at St John's Church (the alleged place of the baptism) was unveiled a few years ago, commemorating the incident in the Czech and German languages.
In 1096, on the way to the First Crusade, Peter the Hermit led a mob of Crusaders who attempted to force the mass conversion of the Jews of Regensburg and killed all those who resisted.
In 1135–1146 a bridge across the Danube, the Steinerne Brücke, was built. This stone bridge opened major international trade routes between Northern Europe and Venice, and this started Regensburg's golden age as a city of wealthy trading families. Regensburg became the cultural center of southern Germany and was celebrated for its gold work and fabrics.
The remains of the East Tower of Porta Praetoria from Ancient Roman
In 1245 Regensburg became a Free Imperial City and was a trade center before the shifting of trade routes in the late Middle Ages. At the end of the 15th century Regensburg became part of the Duchy of Bavaria in 1486, but its independence was restored by the Holy Roman Emperor in 1496.
The city adopted the Protestant Reformation in 1542, and its Town Council remained entirely Lutheran until the incorporation of the city into the Principality of Regensburg under Carl von Dalberg in 1803. A minority of the population stayed Roman Catholic and Roman Catholics were excluded from civil rights ("Bürgerrecht"). The town of Regensburg must not be confused with the Bishopric of Regensburg. Although the Imperial city had adopted the Reformation, the town remained the seat of a Roman Catholic bishop and several abbeys. Three of the latter, St. Emmeram, Niedermünster and Obermünster, were estates of their own within the Holy Roman Empire, meaning that they were granted a seat and a vote at the Imperial diet (Reichstag). So there was the unique situation that the town of Regensburg comprised five independent "states" (in terms of the Holy Roman Empire): the Protestant city itself, the Roman Catholic bishopric and the three monasteries mentioned above.
From 1663 to 1806, the city was the permanent seat of the Reichstag of the Holy Roman Empire. Thus Regensburg was one of the central towns of the Empire, attracting visitors in large numbers. In 1803 the city lost its status as a free city. It was handed over to the Archbishop of Mainz and Archchancellor of the Holy Roman Empire Carl von Dalberg in compensation for Mainz, which had become French under the terms of the Treaty of Lunéville in 1801. The archbishopric of Mainz was formally transferred to Regensburg. Dalberg united the bishopric, the monsteries and the town itself, making up the Principality of Regensburg (Fürstentum Regensburg). Dalberg strictly modernised public life. Most importantly he awarded equal rights to Protestants and Roman Catholics. In 1810 Dalberg ceded Regensburg to the Kingdom of Bavaria, he himself being compensated by the towns of Fulda and Hanau being given to him under the title of "Grand Duke of Frankfurt".
Between April 19 and April 23, 1809, Regensburg was the scene of the Battle of Ratisbon between forces commanded by Baron de Coutaud (the 65th Ligne) and retreating Austrian forces. It was eventually overrun after supplies and ammunition ran out. The city suffered severe damage during the fight with about 150 houses being burnt and others being looted.