The Church of Saint Charles at the Four Fountains
(Chiesa di San Carlo alle Quattro Fontane
also called San Carlino) was designed by the architect Francesco Borromini
(1599-1667) and was his first independent commission.
It is an iconic masterpiece of Baroque architecture, built as part of a complex of buildings on the Quirinal Hill for the Spanish Trinitarians, an order dedicated to the freeing of Christian slaves. He received the commission in 1634, under the patronage of Cardinal Francesco Barberini
, whose palace was across the road. The tight geometric complexity of interlocking ovals and circles creates spaciousness in the small corner church, which stands a stone's throw from the Palazzo Barberini (windows designed by Borromini) and piazza. It is also down the street from rival Gian Lorenzo Bernini's oval Sant'Andrea al Quirinale.
For reference, a plan of the church is available here