Ubi's site about Rome, the Eternal City

A couple of facts provide a lead for understanding the history and the arts of Rome in the Reneaissance: first of all, the power of the Church is strictly bound to the destiny of this incredible place, and, even if hosting magnificent buildings and important families, by some standards the city of Rome was plagued by the poverty of a large part of its citizens along with the wealth of a few, for the whole Medieval time.
So, when the popes moved their base to Avignon, inevitably the population reached its minimal count, as the very existence of an economy was bound to presence of the Church, something that you would not expect for the area that had been the capital of the Roman Empire (some account for one million inhabitants at its maximum splendor).
And when Gregory XI returned to Rome, the city was torn by intestine struggles.

It was Martin V of the Colonna family who could settle things down and recover the normal course of life in the area. Since 1420, he also gave implulse to monumental and artistic efforts and in 1423 a jubilee celebrated the rebirth of the town. At this time, the pope moved to Vatican, and the sorroundings underwent a significant if not gigantic rework: the Renaissance, for Rome, came with an actual resurrection of the city.

A number of artists, including foreign ones and Brunelleschi and Donatello, from Florence, just to mention some, came to the city for studying the Roman arts.
But it was the pope in person who asked for Masaccio and Masolino to come. It is also important to note the effort of Leon Battista Alberti, who produced the Descriptio Urbis Romae, planning a geometric arrangement of the city.

After Martin V, Eugenius IV was another man of culture and one who travelled; he wanted more famous artists to decorate Rome. With the Council of Basel, the monarchical structure of the papacy was restored, this gave boost to the power of the Pope who recovered working on the Roman basilicas.
Filaret ended in 1445 The bronze doors of St. Peter, for instance, so they belong to this time.
It was under Eugenius IV that, finally, some common style elements spread between artists of different provenience; in such period, artists of the importance of Beato Angelico were working in Rome.

Nicholas V (1447-1455) was responsible for a more ambitious plan that also was a start for subsequent interventions; from a general point of view, these included the restoration or reconstruction of forty churches, the restoration of the City walls, the complete reforming of the Borgo and the expansion of St. Peter.
The main intent was to state the power of the Church by providing it with its own citadel, raising above the rest of buildings around; this is precisely the sensation that you can have today by standing in the old Borgo or in piazza Risorgimento and watching the high walls of the Vatican Hill.
Nicholas V wasn't pope for long, but his intentions were also fulfilled by the successors.
Palazzo Venezia ("Venetia Palace") is an important example of the style that was taking life in Rome: it was started in 1455 by incorporating existing buildings, using Roman elements but often in a different scale and for different purposes: the actual production reused ancient models and was inspired by them.

Palazzo Venezia in Rome, on the square by the same name

The Constantinian basilica of St. Peter was renewed by Bernardo Rossellino, who prolonged the body with five aisles; pillars were used to incorporate the old columns and the apse had to be rebuilt because the transept had expanded, a choir was added.
Also the Apostolic Palace underwent significant reworkings; Frate Angelico, Benozzo Gozzoli and other prominent painters decorated the Niccoline Chapel (privately used by the pope); the erudite, rich images fitted the large architecture.
In general, even if some common elements can be traced in most artists' works, defining the developing "Roman" style, the number of them and the different origins granted quite a variety of results.

As for Pius II (1458-1464), it is important to remind that under his reign Piero della Francesca was active for the production of frescoes that -regretfully- have been lost while Sixtus IV (1471-1484) is renowed for the creation of the Vatican Library; Melozzo da Forlì obtained the important task of painting the frescoes.
Some time later, the same Melozzo da Forlì (Giuliano della Rovere was the commissioner, this time) painted the apse of the Basilica dei Santi Apostoli, where the Ascension of the Apostles between Playing Angels is considered the first example of an intentional view "from down to top".

Around 1480, the building of the world-known Sistine Chapel took place. Lorenzo de' Medici of Florence obtained that the decoration was commissioned to prominent artists from Florence, such as Perugino, Ghirlandaio and Botticelli. This was not simply an act of generosity; on the contrary, it had a political significance and was meant to increase the influence of the Tuscan city.
Since then, the Chapel represented an example for all the Renaissance artits to come.

Pope Alexander VI came from the powerful Borgia family, of Spanish origin; Pinturicchio is the most relevant painter of the Renaissance that worked for him.

Pinturicchio: Resurrection of Christ with pope Alexander VI on his knees, in the Vatican residence

Alexander VI ruled the Church to the year 1503, and by the time all of the Renaissance features were fully expressed in the art of Rome, and for long more, it was the style that permeated all of the creations and the architecture in the Eternal City.