Legend says that pubescent Agnes was cast naked into the stadium of Domitian around AD 304 when she refused to renounce Christ and marry some powerful local buck (possibly Romulus, the son of Emperor Maxentius), but her hair grew miraculously to save her from embarrassment. She was condemned to burn but when the fire refused to catch light, her pagan persecutors chopped her head off, supposedly on the exact spot where this church now stands.
Carlo and Girolamo Rainaldi began the church for Pope Innocent X in 1652; after they quarrelled with the pope, Borromini was appointed in their place. He revised the design considerably, adding the splendidly fluid concave façade. The trompe l'oeil side chapels contain statues of St Agnes and another victim of a botched martyrdom, St Sebastian. The doorway in the chapel to the left of the high altar contains a reliquary with Agnes's implausibly small skull. Around the church are reliefs with cherubs holding symbols associated with the martyrdom, including the lamb most commonly used to represent her. Admire Borromini's sacristy during a Sunday early-evening concert.