The city was probably created ex nihilo in the early 1st century AD, as the capital of the recently conquered territory of the Helvetii, across the road that connected Italy to Britain, built under Claudius. Under the rule of Emperor Vespasian, who grew up there, Aventicum was raised to the status of a colony in 72 AD, whereupon it entered its golden age. The town wall was 5.6 km (3.48 miles) long.
In the Christian era Aventicum was the seat of a bishopric. The most famous of its bishops was Marius Aventicensis, whose terse chronicle, spanning the years 455 to 581, is one of the few sources for the 6th-century Burgundians. Shortly after the Council of Macon, 585, Marius removed the see from Aventicum, which was rapidly declining, to Lausanne.