The remains of a 10,000-seat amphitheatre, built in the second century AD and quarried for stone after antiquity, sit a short walk east of the old town near the sea. Only a section of the outer arcade and a fragment of the ellipse survive, but enough to give a sense of the original scale.
The site is fenced but visible from the surrounding streets; it is occasionally opened for summer concerts and theatre, which is the best way to experience it. Combine with a walk back into the historic centre along Via Tonini.