From the colony to the fort: late antiquity on the Rhine

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The prosperity of Augusta Raurica did not last. From the middle of the third century AD, the Roman frontier on the upper Rhine and the Danube came under pressure. When the Romans abandoned the territories beyond the Rhine and the Danube, the Agri Decumates, around 260, the Rhine once again became the frontier of the Empire. Augusta Raurica suffered from Alemannic incursions and from the internal crises of the Empire. The upper town was partly abandoned and a fortification was built on the plateau of Kastelen.

Around 300 AD, under the Tetrarchy, a new fort, the Castrum Rauracense, was built on the bank of the Rhine at today's Kaiseraugst. The fort became the military and administrative centre of the region and, in the fourth century, the seat of a garrison of the legio I Martia.¹ The civil population concentrated within its walls or in its immediate vicinity. At the same time, the Münsterhügel in Basel was fortified again, with a small late Roman fort occupying the old Celtic site.

In this period the name of Basel appears for the first time in written sources. The historian Ammianus Marcellinus reports that in 374 the emperor Valentinian I had a fortification built near Basilia, which the local inhabitants called Robur.² The name Basilia is probably derived from a Greek or Latin personal name or from the Greek basileia, "royal"; its exact origin remains uncertain. The appearance of the name in the context of a military building programme is characteristic: in late antiquity Basel was a frontier post, part of the chain of forts and watchtowers that the Romans built along the Rhine from Lake Constance to the area of Basel in the fourth century. Several of these watchtowers have been found along the Swiss bank of the Rhine.

Christianity reached the region in the fourth century. A bishop of the Rauraci is mentioned in a list attributed to a council of Cologne of 346, whose authenticity is disputed. The episcopal seat seems originally to have been located in the Castrum Rauracense and was later transferred to Basel, where the bishops are attested with certainty from the eighth century.³ The transfer of the bishopric marks the beginning of Basel's history as the dominant town of the region.

Notes

1. On the Castrum Rauracense and the legio I Martia, see HLS, articles "Kaiseraugst" and "Augusta Raurica"; Berger, Führer durch Augusta Raurica, 2012. ↩
2. Ammianus Marcellinus, Res gestae XXX.3.1: Valentinian builds a munimentum "prope Basiliam, quod appellant accolae Robur". The identification of Robur is disputed; see HLS, article "Basel (Gemeinde)". ↩
3. On the list of bishops attributed to the Council of Cologne (346) and the transfer of the episcopal see from Kaiseraugst to Basel, see HLS, article "Basel (Fürstbistum)" and "Basel (Diözese)". The authenticity of the Cologne list is disputed. ↩

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