English: Looking from the colonnade to the east of the Theatre Baths we look towards the Tetrastoon proper.

The Tetrastoon, a piazza with four porticos and an altar at the centre, one could get from there to the theatre, which is adjacent. One supposes it was used as a market and shopping street and was built in the fourth century AD when the older agora was permanently flooded because of the rising level of groundwater.

Wiktionary has "A four-sided colonnade, typically enclosing a courtyard" and "From Ancient Greek τετράστῳον (tetrástōion), from neuter of τετράστῳος (tetrástōios, “having four porticoes”) from Ancient Greek τετράς (tetrás, “four”) + στοά (stoá, “portico, colonnade”)"

From the Excavations website I quote: "The marble-paved and colonnaded square in front of the Theatre, known as the Tetrastoon (Four Porticoes), was built in the first or second century AD, but its current form is due to a restoration in the AD 360s by a provincial governor called Antonius Tatianus. The east side of the square became at this time a place of honour for newly erected portrait statues of emperors and governors. Several were set up in the later fourth and fifth centuries. Two of these statues survive, one for Theodosius I or II and one for a governor called Flavius Palmatus. "