The Palace of Antiochos was an aristocratic house located west of the Hippodrome and south of the Mese. It was built in the first half of the 5th century by Antiochus, a Persian eunuch influential in the court of Theodosius. North of this palace is another palace, commonly identified as the Palace of Lausus. Part of the palace was later converted into the Church of Hagia Euphemia. 

Antiochus was an influential Persian eunuch in the court of Theodosius II (402-450). He probably came to Constantinople as result of an agreement between Emperor Arcadius (395-408) and the Persian king Yazdegerd I (399-420), which made Yazdegerd the guardian of Arcadius’ young son Theodosius II. Antiochus probably arrived in Constantinople soon after 402 and first acts as cubicularius (chamberlain) and tutor of Theodosius II. At some point after Arcadius’ death, the eunuch attained the post of praepositus sacri cubiculi (Grand Chamberlain). When he stepped down from the post, he was made a patrician and continued to be influential. At a later point, he lost favor with Theodosius, was forced to become a priest and all of his property was confiscated (after which the palace became an imperial possession).  The date of his final fall from power is uncertain. It could have been connected to Pulcheria, sister of Theodosius II, after she was proclaimed Augusta in 414. However it is possible his career was briefly interrupted and that his final fall from imperial favor was as late as 439. His palace was seemingly at some point between around 408 and 439. Evidence from brickstamps suggests it was built sometime after 429 before Antiochus’ fall from power. 
The Palace of Antiochus is one of the rare examples of an aristocratic house in Constantinople to have survived. While around 50 aristocratic houses from the 4th and 5th centuries are recorded in written sources, very few have actually been discovered. The Palace of Antiochus and another nearby palace (probably incorrectly identified as the Palace of Lausus) are the only two that have been excavated. The first remains discovered on the site in 1939 were frescoes on the remains of a wall, which allowed for its identification as the Church of St. Euphemia.  In 1942, a hexagonal hall with a semicircular portico was revealed. During excavations in 1951-52, a column base was found in situ in the portico, which had the inscription “Of Antiochus the praepositus” allowing for it to securely be identified. Excavations uncovered the vault of a channel under the sigma-plan colonnade that led from the Binbirdirek Cistern.

The Palace of Antiochus consisted of a hexagonal hall with a sigma-plan (semicircular) portico. The hexagonal structure – probably served as a dining hall (triclinium) - had a diagonal around 20 meters, with an apse on each side of the hexagon, except for the side with the entrance. Each apse had dimensions that allowed for a semicircular bench (stibadium) and a dining table to be installed there. Small circular rooms were inserted between two neighboring apses and communicated with some of them. There was a marble pool in the floor in the center of the hall. The hexagon was preceded by a wide semicircular portico. Further smaller rooms of centralized plans were grouped along the back wall of the semicircular portico and these may have served as bedchambers (cubicula) and/or private dining rooms. The semicircular portico had a diameter of about 60 meters and was paved with marble slabs. The palace was entered from a street perpendicular to the north side of the hippodrome, accessible by means of an elegant semicircular portico and garden. Symmetrically disposed pavilions of intricate geometry opened onto the portico. Small colonnaded porticoes opened outward to the surrounding gardens.