Dating to the 1st century BC, this great drum of a mausoleum encloses a burial chamber, now roofless. In the 14th century it was converted into a fort by the Caetani family, who were related to Pope Boniface VIII, and used to frighten passing traffic into paying a toll. Amid the interesting displays of artefacts inside, look for the coin with Nero's head from 64–66 AD.
The tomb was built for the daughter of the consul Quintus Metellus Creticus. Cecilia Metella was of particular significance as she joined two important families by marriage – she was also daughter-in-law of Crassus, Julius Caesar's banker. The walls are made of travertine and the interior is decorated with a sculpted frieze featuring Gaelic shields, ox skulls and festoons.