The atrium's external portico comprises 16 2nd-century Corinthian columns, perhaps from a 2nd-century pagan temple.

The Early Christian church of S Lorenzo, remodelled in the 12th and 16th centuries, was stripped during the 1930s to expose the surviving original fabric. The date of its foundation is disputed, whether in the later 4th or early 5th century, and its original dedication is unknown. The basic plan of the church is a quatrefoil of so-called double-shell design. Most double-shell churches built between the late 4th century and the 7th were tetraconchs. S Lorenzo is one of only two examples in Italy. The atrium to the west and the two chapels of SS Ippolito and Aquilino, which adjoin the east and south lobes of the tetraconch respectively, also form part of the original complex. The atrium's external portico comprises 16 2nd-century Corinthian columns, perhaps from a 2nd-century pagan temple. The chapels are both octagonal.