Sant'Agnese in Agone (Piazza Navona)

On Piazza Navona, with a concave Baroque facade by Borromini (1652) — designed in deliberate dialogue with the curves of Bernini's Four Rivers Fountain directly opposite. Built on the supposed site of Saint Agnes's martyrdom — she was a 13-year-old girl killed during Diocletian's persecution in 304 AD, and her skull is preserved in a chapel inside. The interior, with frescoed dome, is more conventional than Borromini's exterior. Steps inside lead down to the original early-medieval church and the supposed site of her execution.