The Augustaion (or Augusteum) was enclosed open space in Constantinople, situated south of Hagia Sophia. While the Augustaion changed over time, it was of great ideological and symbolic important, in part due to its connection with Hagia Sophia and the Great Palace.
The Augustaion was a central location in Constantinople with many key monuments located in the vicinity. During the Constantinian era, it was a large open area adjacent to Hagia Sophia to the north, the Senate House and the Great Palace to the east, the Baths of Zeuxippus to the south, and the Basilica (now marked by the Basilica Cistern) to the west. Just outside the Augustaion was the Milion, the milestone marking the beginning of the Mese, the great colonnaded main street of Constantinople. Later the the Patriarchate was built between Hagia Sophia and the Augustaion.
Originally an agora known as the Tetrastoon, it is reportedly one of the older structures dating to the reign of Septimius Severus rebuilt or remodeled by Constantine. It was renamed in honor of Helena (the mother of Constantine the Great), who had been given the title Augusta. Constantine is said to have placed in it a statue of his mother Helena on a porphyry column.
The region was severed damaged by fire during the Nika Riots in 532, requiring Justinian to rebuild the Augustaion, the Baths of Zeuxippus, the Senate House and Hagia Sophia. The Augustaion was subsequently considerably different from the original ceremonial space laid out by Constantine. It was now smaller and served as a restricted courtyard to the church of Hagia Sophia rather than as a public forum. The Senate House was also rebuilt by Justinian with a porch of six huge marble columns.
The main feature of the Augustaion was a column crowned with a bronze statue erected in 543. In front of this column stood the statues of three barbarian kings in a posture of submission. This column replaced an earlier column with an equestrian statue of Theodosius I. There was also a column with a silver statue of Aelia Eudoxia (the wife of Arcadius) that seems to have been erected on the northeast side of the Augustaion, outside the Senate House. Its base has survived and is now located in the courtyard of Hagia Sophia.
Two Romes: Rome and Constantinople in Late Antiquity edited by Grig and Kelly
Brickstamps of Constantinople by J. Bardill
The Cambridge Companion to the Age of Justinian edited by M. Maas
Oxford University Press Dictionary of Byzantium
Augustaion (Byzantine 1200)