Santo Stefano Rotondo al Celio, which was built during the reign of Pope St Simplicius (r. 468-83), is one of the world's oldest and largest circular churches.
When Santo Stefano was originally built it consisted of a series of three circular concentric naves separated by rings of antique columns. Two of these mostly disappeared during the course of restoration in the 15th century, but the inner ring, comprising 22 ancient granite columns, remains intact and is still very impressive.
Santo Stefano is renowned for its late 16th century frescoes, which depict, in much gory detail, the martyrdom of some of the early Christian saints. The frescoes were commissioned by Pope Gregory XIII (r. 1572-85) and the artist was Niccolo Circignani, better known as il Pomarancio.