There are traces of a settlement at the Rhine knee from the early La Tène period (5th century BC). In the 2nd century BC, there was a village of the Raurici at the site of Basel-Gasfabrik, to the northwest of the Old City, likely identical with the town of Arialbinnum mentioned on the Tabula Peutingeriana.[10] The unfortified settlement was abandoned in the 1st century BC in favour of an oppidum on the site of Basel Minster, probably in reaction to the Roman invasion of Gaul.

In Roman Gaul, Augusta Raurica was established some 20 km (12 mi) from Basel as the regional administrative centre, while a castra (fortified camp) was built on the site of the Celtic oppidum. The city of Basel eventually grew around the castra. In AD 83, Basel was incorporated into the Roman province of Germania Superior. Roman control over the area deteriorated in the 3rd century, and Basel became an outpost of the Provincia Maxima Sequanorum formed by Diocletian.

The Germanic confederation of the Alemanni attempted to cross the Rhine several times in the 4th century, but were repelled; one such event was the Battle of Solicinium (368). However, in the great invasion of AD 406, the Alemanni appear to have crossed the Rhine river a final time, conquering and then settling what is today Alsace and a large part of the Swiss Plateau. From that time, Basel has been an Alemannic settlement.

The Duchy of Alemannia fell under Frankish rule in the 6th century, and by the 7th century, the former bishopric of Augusta Raurica was re-established as the Bishopric of Basel. Under bishop Haito, the first cathedral was built on the site of the Roman castle, later replaced by a Romanesque structure consecrated in 1019. At the partition of the Carolingian Empire, Basel was first given to West Francia, but it passed to East Francia with the treaty of Meerssen of 870. The city was plundered and destroyed by a Magyar invasion in 917. The rebuilt city became part of Upper Burgundy, and as such was incorporated into the Holy Roman Empire in 1032.

From the donation by Rudolph III of Burgundy of the Moutier-Grandval Abbey and all its possessions to Bishop Adalbero II of Metz in 999 until the Reformation, Basel was ruled by prince-bishops, whose memory is preserved in the crosier shown on the Basel coat of arms.

In 1019, the construction of the cathedral of Basel (Münster) began under Henry II, Holy Roman Emperor. In 1225–1226, a bridge, now known as the Middle Bridge, was constructed by Bishop Heinrich von Thun and Lesser Basel (Kleinbasel) founded as a bridgehead to protect the bridge. The bridge was largely funded by Basel's Jewish community who had settled there a century earlier. For many centuries to come Basel possessed the only permanent bridge over the river "between Lake Constance and the sea". The Bishop also allowed the furriers to establish a guild in 1226. Eventually about 15 guilds were established in the 13th century. They increased the town's, and hence the bishop's, reputation, influence, and income from the taxes and duties on goods in Basel's expanding market.

In 1412 (or earlier), the well-known Gasthof zum Goldenen Sternen was established. Basel became the focal point of western Christendom during the 15th century Council of Basel (1431–1449), including the 1439 election of antipope Felix V. In 1459, Pope Pius II endowed the University of Basel, where such notables as Erasmus of Rotterdam and Paracelsus later taught. At the same time the new craft of printing was introduced to Basel by apprentices of Johann Gutenberg.

The city had remained neutral through the Swabian War of 1499 despite being plundered by soldiers on both sides. The Treaty of Basel ended the war and granted the Swiss confederates exemptions from the emperor Maximillian's taxes and jurisdictions, separating Switzerland de facto from the Holy Roman Empire.

On 9 June 1501, Basel joined the Swiss Confederation as its eleventh canton. It was the only canton that was asked to join, not the other way round. Basel had a strategic location, good relations with Strasbourg and Mulhouse, and control of the corn imports from Alsace, whereas the Swiss lands were becoming overpopulated and had few resources. A provision of the Charter accepting Basel required that in conflicts among the other cantons it was to stay neutral and offer its services for mediation.

In 1529, the city became Protestant under Oecolampadius and the bishop's seat was moved to Porrentruy. The bishop's crook was however retained as the city's coat of arms. For centuries to come, a handful of wealthy families collectively referred to as the "Daig" played a pivotal role in city affairs as they gradually established themselves as a de facto city aristocracy.

Swiss History Timeline - Basel (City)
Basel (City) coat-of-arms