Built in the early 14th century on the alleged site of the martyrdom of Saint Ercolano (occurred during the siege of Totila, 548 A.D.), it features an unusual octagonal-shaped tower. Originally consisting of two floors, the second floor was demolished in the period of construction of the nearby Rocca Paolina (1540-43). In 1604 the original semicircular stairway was replaced with the current double one. From the 17th century is also the redecoration of the interior, with frescoes by Andrea Carlone and Nicola Giuli, and stuccoes in the chapels, by Jean Regnaud, known as Sciampagna (1682). Noteworthy is the ancient Roman sarcophagus (3rd cent. A.D.) used as base for the main altar. It is sculpted with spiral fluting and hunting scenes and contains the remains of the Saint. Today the church is memorial chapel for the fallen in wars. At the feet of the stairway, a 19th century garden showcases the Neptune fountain originally located in Piazza del Sopramuro (today Piazza Matteotti).