Forum Piscarium ruins, three re-erected columns of the Temple of Apollo Sosianus to right, Theatre of Marcellus in background.

The Forum Piscarium was the fish forum venalium of ancient Rome, north of the Roman Forum, between the Sacra Via and the Argiletum. It was burned in 210 BC and rebuilt the next year. In 179 BC it was incorporated in the general Macellum, built by Marcus Fulvius Nobilior in the same region.

The Temple of Apollo Sosianus (previously known as the Apollinar and the temple of Apollo Medicus) is a Roman temple dedicated to Apollo in the Campus Martius, next to the Theatre of Marcellus and the Porticus Octaviae, in Rome, Italy. Its present name derives from that of its final rebuilder, Gaius Sosius. The three re-erected columns of the temple which survive to full-height today belong to the Augustan rebuild, but the cult of Apollo had existed in this area since at least to the mid-5th century BC. when an Apollinar (a sacred grove or altar) was recorded on this site. The temple's ruins were occupied in the post-Roman period by medieval dwellings right up to the 1930s, when these were demolished between 1926 and 1932 to allow the theatre of Marcellus to be seen in isolation. In those same works the remains of the fallen colonnade were recovered in the very positions in which they had fallen, inside the arches of the theatre and as a result in 1937 and 1938 the podium's remains were excavated. In 1940 the fallen columns were raised on this podium, though probably not in their original positions.

The Theatre of Marcellus (Latin: Theatrum Marcelli, Italian: Teatro di Marcello) is an ancient D-shaped open-air theatre in Rome, Italy, built in the closing years of the Roman Republic. At the theatre, locals and visitors alike were able to watch performances of drama and song. Today its ancient edifice in the rione of Sant'Angelo, Rome, once again provides one of the city's many popular spectacles or tourist sites. Space for the theatre was cleared by Julius Caesar, who was murdered before its construction could begin; the theatre was so far advanced by 17 BC. that part of the celebration of the ludi saeculares took place within the theatre; it was completed in 13 BC and formally inaugurated in 12 BC by Augustus.
The theatre was 111 m in diameter and was the largest and most important theatre in Ancient Rome; it could originally hold between 11,000 and 20,000 spectators.
The theatre fell out of use in the early 4th century and the structure served as quarry for e.g. the Pons Cestius in 370 AD. However, the statues located inside the building were restored by Petronius Maximus in 421 and the remaining structure still housed small residential buildings. In the Early Middle Ages the theatre was used as a fortress of the Fabii and then at the end of the 11th century (when it was known as templum Marcelli), by Pier Leoni and later his heirs (the Pierleoni). This saved the complex from further destruction. The Savelli held it in the 13th century. Later, in the 16th century, the residence of the Orsini, designed by Baldassare Peruzzi, was built atop the ruins of the ancient theatre.

more on https://en.wikipedia.org/wiki/Temple_of_Apollo_Sosianus , https://en.wikipedia.org/wiki/Theatre_of_Marcellus and https://en.wikipedia.org/wiki/Forum_Piscarium