The columns, arches and fragmented pediment of this quadriporto (four-sided porch), which dates from 23 BC, once formed part of a vast rectangular portico supported by 300 columns and extending 132m by 119m. Named after the sister of the emperor Augustus, during whose reign it was constructed, it was heavily restored in the 3rd century and housed Rome's fish market from the Middle Ages until the 19th century.
Behind the portico is the 8th-century Chiesa di Sant'Angelo.