La St Peter's Basilica is located in the Vatican City, an independent sovereign state on the right bank of the Tiber, within RomeTiny in size, the Vatican State is what remains of the Church's temporal dominions, which were annexed to united Italy at the end of the 19th century.
The Basilica as we see it today, with its ribbed dome towering imposingly and the square that seems to welcome all the faithful of the world into the embrace of Mother Church, is the work of the most distinguished architects and geniuses of the Renaissance and Baroque periods, and rests on the foundations of the Constantinian basilica, which lasted over a thousand years and which in turn rested on a sacred area of pagan-Christian mausoleums.
St. Peter's Square with its famous colonnade, one of Gian Lorenzo Bernini's most ingenious inventions, is 320 metres deep with a central ellipse of 240 metres and is surrounded by four rows of 284 columns and 88 pillars. The balustrade above the columns is decorated with 140 statues of saints. At the bottom is an enormous three-tiered staircase with statues of St Peter and St Paul on either side. In the middle of the square are two large fountains and the obelisk.
The façade of the basilica, 114.69 m wide and 47.3 m high, made of Tivoli travertine, has a single order of columns and Corinthian pilasters framing a large central portico with two arches on either side (the left one gives access to the Vatican City); above are nine balconies with windows (the central one is the Loggia of Blessings) and a canonical attic surmounted by a balustrade supporting thirteen statues. Five bronze doors lead into the interior. Above it all is the grandiose 'Michelangelo dome' and the smaller domes of the Gregorian and Clementine chapels.
Inside, the Basilica is 186 m long (218.7 with the portico), the height of the main nave is 46 m, the height of the hollow of the dome is 119 m. Under the dome is the papal altar on which Bernini's famous canopy stands.
The sumptuousness of the interior leaves one breathless: 45 altars, 11 chapels, some 10,000 square metres of mosaics and numerous other works of art such as Michelangelo's Pieta .
Below St. Peter's Church are the tombs of numerous popes.
It was Constantine, the first Christian emperor, who wanted a basilica built in 315 A.D. on the exact spot where the tomb of Christ's first Apostle was venerated.
The Vatican area was originally unhealthy and sparsely inhabited. Its conditions improved at the beginning of the 1st century, when the part closest to the Tiber was reclaimed. Later gardens, vast parks, villas and some large buildings were built, such as the Naumachia Vaticana , probably used for water games, Hadrian's Mausoleum , today Castel Sant'Angelo , and Caligula's private circus . Along the Via Cornelia instead stood sepulchres, are and funerary cippus, in compliance with a strict Roman law that wanted all burial places outside built-up areas.
Built by the Emperor Caligula between 37 and 40 A.D., it stood on the left flank of the present basilica, in the valley leading down to the river. The circus was to be the scene of the first Christian persecution by Nero . The location of the circus had been known since at least the 17th century, also due to the fact that the obelisk that stood in its centre remained in its original place until 1586, when it was moved to the centre of St Peter's Square at the behest of Sixtus V .
Domenico Fontana recounted the event in a book, Della Trasportatione dell'Obelisco Vaticano et delle Fabriche di Nostro Signore Papa Sisto V, Roma 1590 . It took 40,000 scudi of expenditure, 800 workers, 140 horses, 40 winches; Sixtus V issued an edict punishing with death anyone who obstructed the work, or even just made noise: the lifting operations, in absolute silence, had to be accompanied only by the sound of a trumpet.  It is said that Fontana gave the order to have his horse ready to flee in case the obelisk collapsed. Fontana succeeded thanks to one of the workers, sailor Bresca, who, sensing that the support ropes were about to break, broke the silence with a desperate: "water to the ropes!".
To witness the radical transformation of the entire Vatican area, one has to go back to the 4th century, when Christianity rapidly imposed itself on pagan cults. After the abdication of Diocletian, author of the last great persecution, the succession struggles for the conquest of power saw the appointment of Constantine as emperor in 307. The son of a general of Diocletian, Constantine was definitively recognised in his office in 312, when near Rome, at Saxa Rubra, on 28 October he defeated the army of his rival Maxentius, who drowned in the Tiber. The following year, with the Edict of Milan, the Emperor established the liberalisation of religion, so that Christianity was no longer hindered and could be freely professed.
From then on, political and religious power were no longer unified in the single person of the emperor, to the point that in 330 he moved the capital to the East, founding a city named after himself on the Bosporus: Constantinople. Instead, he made Rome the religious centre of the Empire, and to this end he initiated an intensive building programme that was to provide the nascent Church with suitable seats. First of all, a basilica was built to properly celebrate the prince of the apostles. Then that of St John Lateran with the Pontiff's residence and the imperial palace; then St Cross in Jerusalem, St Peter and Marcellinus, St Sebastian, St Lawrence Outside the Walls and finally St Agnes .
La St Peter's Basilica grew also thanks to interventions and donations by princes and pontiffs; in 800 Charlemagne was crowned there by Leo III°, and after him Lothair, Ludwig II° and Frederick III°
On the threshold of the early Middle Ages there is the progressive decline of the city of Rome, by then no longer the fulcrum of a great empire but the target of pillage for the barbarian hordes, from the Goths of Alaric (410) and Vitiges (537-538), to the Vandals of Genseric (445), who cut off the aqueducts to bend the city, and finally Totila
One thousand years after its foundation, St Peter's was falling into ruin and it was Nicholas V° who renewed and began the extension of the basilica at the suggestion of Leon Battista Alberti and to a design by Bernardo Rossellino. During the Renaissance there was a new cultural and political climate in Italy and Europe, the rebuilding of Rome began (the urban situation at the time and the transformations) on the initiative of a new generation of popes who saw building works as a means of reaching the masses, whom Nicholas V, the humanist pope, saw as needing to be fascinated by grandiose spectacles.
And here is the magnificent plan of Nicholas V, the restoration of ancient monuments that could be used as infrastructures of the papal city: the Aurelian walls, the bridges, Hadrian's mausoleum transformed into a castle, some aqueducts, the reconstruction or repair of the forty basilicas that constituted the Holy Stations of pilgrimage, and finally the creation of a citadel on the Vatican hill, imagined as a holy city distinct from the profane one, beyond the Tiber communicating only through the hinge of Castel S. Angelo .
Nicholas V was only able to realise his project to a small extent. It was up to Julius II della Rovere to build the new basilica. It began with the demolition of a large part of the old church by Bramante, with the intention of constructing a building with a Greek cross plan inspired by the Pantheon.
Of Bramante's design, the central pillars were built, with the arches supporting the dome, and the spaces adjacent to the central core were set up, then work stopped for 20 years. In 1527, among other things, there was the terrible sack of Rome by the Lansquenets.
The work was then directed by Frà Giocondo , Raphael , Giuliano da Sangallo , Baldassarre Peruzzi , Antonio da Sangallo the Younger and finally Michelangelo , who took up Bramante's plan, restructuring the smaller spaces surrounding the central core and beginning the construction of the dome, which was only finished under Sixtus V in 1593 by Giacomo Della Porta and Domenico Fontana .
Under the pontificate of Paul V it was decided to restore the basilica layout with the definitive return to the Latin cross. The architect Carlo Maderno added three chapels on each side to the building and led the naves up to today's façade (begun in 1607 and finished in 1614) restored for the Jubilee of 2000 and criticised by many because, by hiding the drum, it dampens the ascending effect of the dome. The consecration of the new basilica was celebrated by Urban VIII in November 1626.
Once the great work was completed, the building of the city came to a halt, but the miraculous balance between ancient ruins and the Baroque scenery of papal Rome was such that it fascinated and enraptured the great travellers: Byron, Goethe, Stendhal .