Marie-Lan Nguyen [public domain], via Wikimedia Commons
Sometimes identified by the diminutive San Carlino because of its tiny size, this is one of Borromini's masterpieces. In a space no larger than the base of one of the piers of St. Peter's Basilica, he created a church that is an intricate exercise in geometric perfection, with a coffered dome that seems to float above the curves of the walls. Borromini's work is often bizarre, definitely intellectual, and intensely concerned with pure form. In San Carlo, he invented an original treatment of space that creates an effect of rippling movement, especially evident in the double-S curves of the facade. Characteristically, the interior decoration is subdued, in white stucco with no more than a few touches of gilding, so as not to distract from the form. Don't miss the cloister, a tiny, understated baroque jewel, with a graceful portico and loggia above, echoing the lines of the church.