The Altar of Hieron is a monumental grand altar in the ancient quarter of Neapolis in Syracuse. It was built in the Hellenistic period by King Hiero II and is the largest altar known from antiquity.
The structure is aligned roughly north-north-west to south-east-east, and is located in the Neapolis. Almost nothing except the foundations of the structure survive today. The structure was partly built from masonry blocks and partially carved from the living bedrock. The altar itself is 20.85 metres wide and 196 metres long (exactly one Doric stade). This makes it the largest altar known from the ancient world.
The altar is mentioned by Diodorus Siculus, who attributes its construction to Hiero II. Stylistic analysis of the sculptural fragments from the altar confirms this, showing that they were made at the same time as the third phase of the nearby Greek theatre, which belongs after 235 BC. The votive deposit in the natural grotto under the eastern side of the altar shows that the area was already a sacred site in the Archaic period, not long after the city of Syracuse was established.
References:The ancient Argos Theater was built in 320 BC. and is located in Argos, Greece against Larissa Hill. Nearby from this site is Agora, Roman Odeon, and the Baths of Argos. The theater is one of the largest architectural developments in Greece and was renovated in ca 120 AD.
The Hellenistic theater at Argos is cut into the hillside of the Larisa, with 90 steps up a steep incline, forming a narrow rectilinear cavea. Among the largest theaters in Greece, it held about 20,000 spectators and is divided by two landings into three horizontal sections. Staircases further divide the cavea into four cunei, corresponding to the tribes of Argos A high wall was erected to prevent unauthorized access into the theatron and may have helped the acoustics, but it is said the sound quality is still very good today.
Around 120 CE, both theaters were renovated in the Roman style.