From the 6th century AD a first palaeo-Christian basilica existed here, having three naves with a central apse, five meters under the current cathedral.
After the destruction of the ancient Canosa by Muslim raiders, numerous clerics moved to the Barlettan church, which was renamed as Santa Maria de Auxilio: the 9th century structure had a Latin cross plan, with a pavement mosaic of which traces exist today.
In the 13th century the Palatine Count Giovanni Pipino da Barletta, a friend of king Charles I of Anjou, promoted a further expansion of the church.
The bell tower is located on the northern side, and gives access to the castle through an archway under which are remains of the ancient church's pavement.
Some bas-reliefs from the original portal are inside the cathedral, portraying scenes of the Last Supper and the entrance of Christ into Jerusalem.