A cramped, two-level stone dungeon beside the Forum where, by tradition, Saints Peter and Paul were held — you descend a tight stair into a windowless lower cell, cold and low-ceilinged, with the spring said to have appeared for Peter's baptisms. The multimedia layer helps, but it's brief and steep for the price: genuinely affecting if the saints' story or grim Republican-era history moves you, an easy skip if not.