Piazza Navona was built on the site of the 1th-century Stadium of Domitian and follows the form of the open space of the stadium. Defined as a public space in the last years of 15th century, when the city market was transferred there from the Campidoglio, Piazza Navona was transformed into a highly significant example of Baroque Roman architecture and art during the pontificate of Pope Innocent X. He reigned from 1644 until 1655, and whose family palace, the Palazzo Pamphili, faced the piazza. It features important sculptural and architectural creations like the famous Fontana dei Quattro Fiumi (1651) by Gian Lorenzo Bernini, topped by the Obelisk of Domitian, brought in pieces from the Circus of Maxentius.
Piazza Navona has two other fountains. At the southern end is the Fontana del Moro with a basin and four Tritons sculpted by Giacomo della Porta (1575) to which, in 1673, Bernini added a statue of a Moor, or African, wrestling with a dolphin. At the northern end is the Fountain of Neptune (1574) also created by Giacomo della Porta; the statue of Neptune, by Antonio Della Bitta, was added in 1878 to create a balance with La Fontana del Moro.
During its history, the piazza has hosted theatrical events and other ephemeral activities. From 1652 until 1866, when the festival was suppressed, it was flooded on every Saturday and Sunday in August in elaborate celebrations of the Pamphilj family. The pavement level was raised in the 19th century, and in 1869 the market was moved to the nearby Campo de' Fiori. A Christmas market is held in the piazza.References:
The Arch of Constantine is situated between the Colosseum and the Palatine Hill. It was erected by the Roman Senate to commemorate Constantine I's victory over Maxentius at the Battle of Milvian Bridge in 312. Dedicated in 315, it is the largest Roman triumphal arch. The arch spans the Via triumphalis, the way taken by the emperors when they entered the city in triumph.
Though dedicated to Constantine, much of the decorative material incorporated earlier work from the time of the emperors Trajan (98-117), Hadrian (117-138) and Marcus Aurelius (161-180), and is thus a collage. The last of the existing triumphal arches in Rome, it is also the only one to make extensive use of spolia, reusing several major reliefs from 2nd century imperial monuments, which give a striking and famous stylistic contrast to the sculpture newly created for the arch.
The arch is 21 m high, 25.9 m wide and 7.4 m deep. Above the archways is placed the attic, composed of brickwork reveted (faced) with marble. A staircase within the arch is entered from a door at some height from the ground, on the west side, facing the Palatine Hill. The general design with a main part structured by detached columns and an attic with the main inscription above is modelled after the example of the Arch of Septimius Severus on the Roman Forum.