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 Petersberg Citadel is one of the largest extant early-modern citadels in Europe and covers the whole north-western part of the Erfurt city centre. It was built after 1665 on Petersberg hill and was in military use until 1963. It dates from a time when Erfurt was ruled by the Electors of Mainz and is a unique example of the European style of fortress construction. Beneath the citadel is an underground maze of passageways that can be visited on guided tours organised by Erfurt Tourist Office.
The citadel was originally built on the site of a medieval Benedictine Monastery and the earliest parts of the complex date from the 12th century. Erfurt has also been ruled by Sweden, Prussia, Napoleon, the German Empire, the Nazis, and post-World War II Soviet occupying forces, and it was part of the German Democratic Republic (East Germany). All of these regimes used Petersberg Citadel and had an influence on its development. The baroque fortress was in military use until 1963. Since German reunification in 1990, the citadel has undergone significant restoration and it is now open to the public as a historic site.