The church is famous for having a ruined propylaeum or gateway of the Portico of Octavia as its narthex. The fabric of the ancient gateway hides the frontage of the church.The mysterious mediaeval brick archway, replacing two columns, is well constructed and has a pair of imposts scavenged from Doric pilasters. There are faint traces of frescoes showing coats-of-arms of cardinals on the brickwork of this. Also, in the tympanum of the pediment are remains of 13th century frescoes of St Michael, the Blessed Virgin and St Paul(?).