Great ancient columns and a marble frontispiece, held together with rusting iron braces, now form part of the church of Sant'Angelo in Pescheria. They were originally the entrance of a massive colonnaded square (portico) containing temples and libraries, built in the first century AD by Emperor Augustus and dedicated to his sister Octavia (this, in turn, had been built over a first-century BC square). The mighty structure was decorated with 34 bronzes by Lysippus depicting bellicose events from the life of Alexander the Great; these are long lost.
The isolated columns outside, and the inscription above, date from a later restoration, undertaken by Septimius Severus in AD 213. After lengthy digs and restoration work in the 1990s, a walkway has been opened allowing you to stroll through the forum piscarium - the ancient fish market, which remained in operation hereabouts until medieval times, hence the name of the church - and get a closer look at the massive remains. Atmospheric (if slightly rubbish-strewn) as the place is, there are no explanations of what you're looking at. The walkway continues past a graveyard of broken columns and dumped Corinthian capitals to the Teatro di Marcello, passing by three towering columns that were part of the Temple of Apollo, dating from 433 BC.