The ancient church of Santi Nereo e Achilleo (St Nereus and St Achilleus) was built in the 9th century, but has been much altered since.
The frescoes on the walls of the nave, which were commissioned by Cardinal Baronius, may be the work of Niccolò Circignani (c.1530-c.1598), also known as Il Pomarancio. The fresco cycle depicts scenes from the lives of saints Nereus, Achilleus and Domitilla.
The 9th century mosaics have survived on the apsidal arch, but have disappeared from the apse itself. The fresco in the half-dome of the apse depicts a group of saints venerating the True Cross, while the fresco below depicts Pope St Gregory the Great chairing a church council.