This tremendous theatrical space, centred on the gleaming marble composition of Bernini's Fontana dei Quattro Fiumi (Fountain of the Four Rivers), is the hub of the centro storico. The piazza owes its shape to an ancient athletics stadium, built in AD 86 by Emperor Domitian, which was the scene of sporting events, and at least one martyrdom. Just north of the piazza, at piazza di Tor Sanguigna 16, you can still see remains of the original arena, sunk below street level. These remains are partially visible from the street; they can also be visited on guided tours on Saturdays and Sundays (10am-1pm; phone 06 6710 3819 for bookings and information).
The piazza acquired its current form in the mid-17th century under Pope Innocent X of the Pamphili family. Its western side is dominated by Borromini's façade for the church of Sant'Agnese in Agone and the adjacent Palazzo Pamphili (now the Brazilian embassy), built in 1644-50. The central fountain, finished in 1651, is one of the most extravagant masterpieces designed - though only partly sculpted - by Bernini. Its main figures represent the longest rivers of the four continents known at the time; the Ganges of Asia, Nile of Africa, Danube of Europe and Plata of the Americas, surrounded by geographically appropriate flora and fauna.
The figure of the Nile is veiled, because its source was unknown. For centuries, the story went that Bernini designed it that way to show the river god recoiling in horror from the façade of Sant'Agnese, designed by his arch-rival Borromini; in fact, the church was built after the fountain was finished. The obelisk at the fountain's centre, moved here from the Circus of Maxentius on the Appian Way, was carved in Egypt under the orders of Domitian (the hieroglyphics are a Roman addition describing him as 'eternal pharaoh'). The less spectacular Fontana del Moro is at the southern end of the piazza. The central figure of the Moor was designed by Bernini in 1653, and executed by Antonio Mari.