This elaborate Baroque church and sanctuary in the upper town is one of the most significant religious sites in southern Lazio. Construction began in the 15th century on the site of an earlier chapel; the ornate interior, with its gilded altars, frescoed vaults, and rich marble inlays, reflects the successive waves of patronage and embellishment that continued into the 18th century.
The sanctuary is dedicated to the Annunciation and has been a pilgrimage destination for centuries. The sacristy and treasury hold a collection of liturgical silver and votive offerings accumulated over five hundred years. Pope Pius IX, who took refuge in Gaeta in 1848–1849 when revolution drove him from Rome, worshipped here during his stay — the visit is commemorated inside.