The cathedral was built in the 5th-6th centuries using the remains of a Roman temple with five naves in the city forum.
The portico, which stands at the top of a flight of 30 steps on the podium of the Roman temple, is of artistic interest.
It contains six reused Classical columns with a triumphal arch in the centre, surmounted by an ancient entablature with mosaic decorations including the figures of a winged monster, an eagle, palms, deer, birds, bulls and other things.
Beneath the portico are a funerary basin of the Roman age and pairs of beasts crouching to either side of the bases of the columns.
A further 7 steps lead to the two entrances to the cathedral, of which the principal one is decorated with marble friezes of the Augustan period.