The Terme della Rotonda is a small Roman bath complex from the second century AD, later converted to a Christian church in the medieval period and then partly built over by neighbouring houses. The visible portion includes a circular domed room — the rotunda from which the modern name comes — that was the bath's frigidarium.
The site is open to the public via a small ticket that includes the adjacent Casa di Pirandello. Half an hour, in the centre, well worth a stop for archaeology completists.