Il Gesú
Giacomo della Porta's most famous church is probably the "Chiesa del Gesù" (lit. Church of the Gesu) in Rome. He also participated in St. Peter's Basilica in Vatican (if I'm not mistaken, he worked...
St. Peter's Basilica, see the link below:
St. Peters Bascillica.
St. Peter's Basilica
Rome, Italy. It is considered a separate country.