Sant'Agnese

Originally the Piazza Navona was the site of ancient roman athletic stadium, which was built by Emperor Domitian. The stadium was shaped like an extended horseshoe, which is the shape of the piazza today.  On the exact site where the current church is located today there was an oratory dedicated to St Agnes, which was then rebuilt into a medieval church.

 In 1652 Pope Innocent X commissioned the demolition of the old church except for the remains in the crypt (which held St Agnes’s remains) and the construction of a new church. Originally the architect in charge of the design was Rainaldi, who proposed a Greek cross plan with a dome which had no drum. Two side towers were to be connected by a substantial stair facing the piazza. 

After Rainaldi was already in the building process the Pope relieved him from his work because his plan was so harshly criticized by the public. The Pope then appointed Borromini to take over as the architect in charge of the church’s construction.

Borromini was forced to work around Rainaldi’s ground plan, but he still managed to put his own twist on the church. He eliminated the vestibule and constructed the towers so that the view of the dome would not be obstructed. He also added a high drum to the dome. After Pope Innocent X’s death in 1655 the relationship between Borromini and the papacy became more difficult. He soon resigned and was not able to finish the central gable and the lantern of the dome. Rainaldi was called back to complete the construction.

Sources.
http://en.wikipedia.org/wiki/Sant'Agnese_in_Agone
http://www.santagneseinagone.org/ricostruzione.html
http://romanchurches.wikia.com/wiki/Sant'Agnese_in_Agone