Beneath the Piazza del Comune, accessible through an entrance on Via Portica, lies the original Roman forum of Assisi, excavated in the 19th century and open as the Museo Civico Foro Romano. The visit takes you through the original pavements, the base of the Temple of Minerva, the tribune where magistrates gave judgements, and a series of small chambers with surviving fragments of sculpture.
It is a quick visit — half an hour or so — but the contrast between the modern piazza above and the empty Roman streets below is striking. Combined ticket with the Rocca Maggiore and the Pinacoteca Comunale.