Stadium of Domitian

The Stadium of Domitian, also known as the Circus Agonalis, was located under the present Piazza Navona which follows its outline and incorporates its remains, to the north of the ancient Campus Martius in Rome, Italy. The Stadium was commissioned around AD 80 by Emperor Titus Flavius Domitianus as a gift to the people of Rome and was used almost entirely for athletic contests. In Christian tradition, Agnes of Rome was martyred there.

Source: Wikipedia