One of Italy’s most antique churches with a round plan, probably built during C.V and consecrated to religion by Pope Simplicio at the end of C.V. It is preceded by a porch with five arcades on high antique granite columns, with Corinthian capitals. It was originally composed of a round environment, circumscribed by two concentric ambulatories, consisting of two rows of columns: the external ambulatory was intersected by the four arms of a Greek cross, with four chapels at the respective ends. Pope Innocenzo II, during C. XII, ordered to add the entry porch and the three grandiose transversal arcades of the inner area, using to high granite columns, aiming at supporting the endangered covering. In 1453, Pope Nicolò V ordered the restoration of the church by ordering the reduction of its diameter. Not to be missed out the interior of the small church, absolutely unusual and very particular, rich, along the walls, of frescoes by Pomarancio, Tempesta and others, quite impressive for the representation of the atrocities performed on christian martyrs. One of the chapels contains a byzantine mosaic dated C. VII which portrays two martyrs buried inside the church.