This church is huge and beautiful. The round plan has the altar surrounded by a circle of pillars, a space which is divided in two by a set of three arches on two even bigger pillars. The outer circle is almost empty and makes the overall space huge. I worked out you could seat 2000 people comfortably.
Wikipedia says, "The walls of the church are decorated with numerous frescoes, including those of Niccolò Circignani (Niccolò Pomarancio) and Antonio Tempesta portraying 34 scenes of martyrdom, commissioned by Gregory XIII in the 16th century. Each painting has a titulus or inscription explaining the scene and giving the name of the emperor who ordered the execution, as well as a quotation from the Bible." The frescoes feature flaying, hacking, pouring lead down the throat, boiling in oil, y'know, the classics. Not for those of a nervous disposition.
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