The Sanctuary of Hercules Victor was one of the largest religious complexes of Republican Rome — a vast terraced platform built in the second century BC and visible for miles along the Via Tiburtina. The temple at its centre was Tivoli's principal place of worship and a major attraction for Roman pilgrims.
Much of the substructure survives intact, having been used over the centuries as a paper mill, a barracks, and a power plant. The site reopened to the public in the 2010s after restoration; the scale of the surviving arches and vaulted galleries is genuinely impressive, and the views down to the Aniene gorge add to the appeal.