It was in the neighborhood of this church that Pope Sixtus IV (Francesco della Rovere, Pope from to 1471-84) settled the stone-masons and construction workers that he had imported from Lombardi to work on his many construction projects, which included the Sistine Chapel, the Vatican Library, Ponte Sisto, and "the Trident" -- the three streets that fan out southward from his grand Piazza del Populo. In fact, the Lombards lived right in the middle of the Populo/Trident city-planning exercise. In 1471, Sixtus allowed the Lombards to form a confraternity for mutual protection and insurance, and for care of their members and visitors from Lombardi. The Lombards renamed the church in honor of St. Ambrose, the patron saint of their province -- Ambrose had been Bishop of Milan from 374-397. By 1513 the confraternity had accumulated enough cash from dues and donations to demolish the small old church and start to build a bigger one.
Only a century later, in 1612, they started another much bigger and grander new church on the site to honor Charles Borromeo, who had been declared a saint in 1610. Before Borromeo was posted to Milan as Archbishop, he had lived and worked in the church of St. Ambrose and ministered to plague victims in the adjoining clinic that had been set up to care for sick Lombard pilgrims. There were several "national" clinics and hospitals along the three streets of the "trident", and some of them are still operating. Porta del Populo was the normal entrance for land travelers and many pilgrims arrived exhausted and in perilous condition -- especially during plague years.
At any rate, the 1612 church with one of the longest church names in Rome is what you now see at Via del Corso 437. It took 70 years to complete the construction following the plans of Onofrio Longhi, who acknowledged that his inspiration was the cathedral in Milan where Charles Borromeo was buried. In 1614, Cardinal Federico Borromeo, Charles Borromeo's cousin and his successor as Archbishop of Milan, delivered the basilica's most famous relic, the heart of Charles Borromeo.
Although S. Carlo al Corso is firmly in the Romanesque/Baroque camp it was clearly inspired in form and floor plan by the Gothic Milan Duomo: It is in the shape of a Latin cross with imposing altars at the ends of its vaulted transept; it has a broad central nave and two side aisles (and therefore is a basilica), outside of which are side chapels on each side; and there is a broad semicircular ambulatory around and behind the main altar. The high dome and cupola at the intersection of the transept and the nave is the final work of Pietro da Cortona and is considered by some architectural analysts to be the crowning achievement of the high baroque period. The façade, the last part of the church to be completed (1684), is pure baroque. It was designed by Luigi Omedei, the Cardinal Protector of the Arch-confraternity who was paying for the work, after he rejected a plan by Carlo Rainaldi.
Small mirror-image palazzi were added on the sides of the façade in 1685 to house staff and inmates of the growing Lombard clinic, which today is a large hospital that includes several adjoining structures. The church is set back from and at a slight angle to the street, because there was a row of low and irregular houses in front of it. When Mussolini's crews widened the Via del Corso in the 1930s, those structures were demolished, and the present Largo di S. Carlo was established in front of the church.
Like other province-sponsored churches in Rome, S. Carlo al Corso fell into disrepair and disrepute after the unification of Italy. Provincial money stayed in the north and papal money was hard to find. Finally, after international architectural organizations had declared it an endangered property -- that is, ready to collapse from structural damage caused by water leaking through its roofs -- in the late 1990's, $2 million in Italian state funds were appropriated and added to a few other smaller grants. Reconstruction started in 1999, and the project started with a new roof. Since then, the dome, transept, side aisles, and main altar have been repaired and cleaned, and the nave now is now full of scaffolding: Jacinto Brandi's vault fresco of "The Fall of the Angels", which no one has seen clearly for centuries, is being cleaned and restored.
Work is expected to be completed within the few months, but the church is well worth a visit even now. Like most Rome churches, S. Carlo al Corso is open on weekdays and Saturdays in the morning and again in the afternoon after a long lunch break -- don't try before 4 PM -- and on Sunday mornings for Catholic mass. Brochures in several languages describing the architecture and artwork are available at the door for an IL2000 donation. The easiest way to get there is to walk up Via del Corso toward Piazza del Populo from Piazza Colonna.
There are, of course, Internet links:
From Italycyberguide: http://www.italycyberguide.com/Geography/cities/rome2000/E18.htm
From Romeartlover, including an old Vasi print: http://members.nbci.com/_XMCM/romeartlover/Vasi140.htm
Short history (and links to info on other Rome churches): http://home.online.no/~cnyborg/carlocorso.html
St. Ambrose, Catholic Encyclopedia: http://www.newadvent.org/cathen/01383c.htm
Charles Borromeo: http://www.newadvent.org/cathen/03619a.htm
Two overhead views of the Church (other pix are in pages/links above): http://cidc.library.cornell.edu/romephotos/photos/49.jpg and http://cidc.library.cornell.edu/romephotos/photos/46.jpg