This fascinating museum is located inside the Palazzo Colonna-Barberini, which was built on the highest terrace of the temple to Fortuna. Today the palazzo-museum incorporates some remains of the ancient construction (plexiglas floor tiles show where the columns once stood), while a model on the top floor shows how the temple complex might have been. There's a selection of Republican and Imperial Roman artefacts: art, instruments and objects either found in the area or associated with the worship of Fortuna. But the star exhibit is the second-century BC Nile mosaic, a work admired by Pliny, which came from the most sacred part of the temple (where the cathedral now stands). It is an intricately detailed, bird's-eye representation of the flora and fauna of the flooded banks of the Nile from Ethiopia to Alexandria. Gallant warriors hunt exotic animals, and diners recline while pipers pipe and goddesses preach. If your Greek is good, you'll be able to identify the labelled beasts.
In a niche off the ground-floor entrance hall, look out for the Capitoline Triad, a second-century AD sculpture of Minerva, Jupiter and Juno sitting together on one throne - the only known portrayal of Rome's three tutelary gods together. This sculpture was stolen in 1992, but subsequently salvaged from the murky underworld of stolen artefacts. Your museum ticket is also good for seeing the archeological excavations.