
Odessos was founded by Ionian Greek colonists from Miletus during the years 585–550 BC. The Greeks established their colony on the site of a Thracian settlement with the same name. Both Greeks and Thracians coexisted peacefully in the colony and in its vicinity, and there is strong evidence of bilateral trade relations and cultural interaction. Odessos was governed by the Boule (City Council) and Demos (Assembly). A monumental Doric temple was built during the second half of the 6th century BC. However, until the middle of the 4th century BC, Odessos remained a relatively small town, although its port was quite important for maintaining trade relations between the ancient Greeks and the Thracians. The first fortification wall was built around the middle of the 4th century BC, and the town was besieged by Philip of Macedon in 339 BC.
In 335 BC, Alexander the Great conquered almost the entirety of ancient Thrace, and Odessos became part of the Macedonian Kingdom. During the reign of Lysimachus, the successor of Alexander the Great who proclaimed himself the King of Thrace, the town became an important naval base for the Macedonian troops who were sent on military missions outside Thrace. In 301 BC, 12,000 infantry and 500 cavalry troops departed to Asia Minor from the port of Odessos. During the Hellenistic period, numerous Thracian tumuli were built in the vicinity of Odessos and many rich burials of the local aristocracy were discovered. In that period Odessos minted its own gold, silver and bronze coins, some of them showing Apollo and Theos Megas (the Great God). During the Hellenistic period, many public buildings were constructed, including a theater, temples and a gymnasium. There was a temple of Apollo, which was later reconstructed during the Roman period, and in the late 4th – early 5th century AD, it was transformed into a Christian basilica. During the 2nd – 1st century BC, a sanctuary of the Thracian Horseman, known as Heros Karabazmos, and a temple of Artemis Phosphoros were built in the town. In 72–71 BC, during the Roman military campaign in Thrace, the town suffered. In the middle of the 1st century BC, Odessos was under the political control of the neighboring Thracian king Sadalas. In 55 and 48 BC, during the military raids of the Getic king Burebistas, the fortification walls of the town were demolished, but later they were repaired during the reign of Emperor Tiberius (AD 14–37).

In 28 BC, Odessos was conquered by the Roman army, and in AD 15, it was included in the Roman Province of Moesia, and became its main port on the Black Sea. However, the town kept its autonomy and was governed by the Boule, Demos and two Archonts. Odessos also minted its own colonial bronze coins from the end of the 1st through the middle of the 3rd century AD. In the Early Imperial period, Odessos became a member of the West Pontic Koinon, which was a federation of the five major towns on the Western Black Sea coast. During the Roman period, the size of the population significantly increased and many people from the Eastern Roman provinces settled in the town. Odessos was divided in seven tribes called phylae. A new fortification wall, which protected a larger area, was built, and many public and private buildings were constructed. The Hellenistic theater was reconstructed. There were shipyards in the town and Odessos maintained intensive trade relations with Asia Minor, Greece and Italy, including with the Roman provinces of Syria and Palestina. New cults appeared during the Roman period: the cult of the Emperor, as well as the cults of Asclepius and Hygeia, Dionysus, Dioskouroi, the Graeco-Egyptian god Serapis and the Persian god Mithra. Temples of the Thracian god Darzalas and of Asclepius and Hygeia were built in the town. Gladiatorial games were organized, as well, and major religious festivities were carried out every five years. Initially, at the end of the 1st century BC, the festivities were called Hermaia in the honor of Hermes, but later, during the Early Imperial period, they were renamed Darzaleia in the honor of the Thracian god Darzalas, who became syncretized with the Great God of Odessos. In AD 242, Emperor Gordian III (AD 238–244) visited Odessos, and commemorative bronze coins on the occasion of this important event were minted.

One of the most impressive buildings in Odessos was the public baths building, constructed over an area of 7000 sq. m, thus being among the largest Roman public baths in Europe. In the early 20th century the ruins of the baths attracted the attention of international scholars and were studied by Ernst Kalinka from the University of Innsbruck and by Karel and Hermengild Škorpil, Czech scholars who also contributed to the foundation of the Archaeological Museum in Varna, which nowadays is among the leading museums in Bulgaria with extremely rich collections. The Roman baths were entirely excavated later, between 1959 and 1971. According to a bilingual inscription in Greek and Latin, during the reign of Emperor Antoninus Pius (AD 138–161), the municipality of Odessos and Titus Vitrasius Pollio, who was Legatus of the Province of Moesia Inferior, built the main water-conduit of the town. The construction probably was carried out in AD 157, and subsequently, the building of the baths was launched. The public baths were functional until the end of the 3rd century AD. Nowadays the walls of the baths are preserved up to 18 m in height, while the original building was about 20 m high. The interior of the baths was constructed in the Roman-Corinthian architectural order and was richly decorated with marble veneer on the walls, marble tiles on the floors and mosaics. The public baths in the Roman Empire were not only a place for leisure and bathing, but also a center for social life. The main bath rooms were frigidaria (the rooms for bathing with cold water), tepidaria (the rooms for bathing with warm water) and caldaria (the rooms for bathing with hot water). The main entrances to the baths in Odessos were situated in the eastern and the western ends of the northern façade. The entrances lead to the eastern and the western antechambers, called vestibuli. There was a basilica thermarum situated between both vestibuli, and it served as a public hall for meetings and talks. A statue of Claudius Aquila, a magistrate of Odessos, who organized the famous religious festivities Darzaleia, was standing inside the basilica thermarum. Marble statues of Heracles, Victoria and Mercury were also found in the baths. The vestibuli were followed by eastern and western dressing rooms, called apodyteria. The visitors left their clothes and valuables in the apodyteria to be kept by their slaves, or by special personnel called capsarii. According to the Roman architectural canon, the frigidaria were situated in the northern part of the public baths, while the tepidaria and the caldaria were situated in the southern part of the building. The bathing procedure started with frigidaria and continued through tepidaria, where the visitors got prepared for bathing with hot water in caldaria. After the bathing, the visitors returned to apodyteria to collect their clothes and valuables. The washrooms, called latrina, were situated in the outer western subterranean gallery. There were also small shops situated in the northern, eastern and western galleries of the baths. In addition, there were shops, hotels and brothels situated around the baths, providing different services for the visitors. Nowadays the Roman baths of Odessos are well restored and are the main archaeological landmark in Varna, which attracts many tourists.

In the middle of the 3rd century AD, the Goths invaded the Roman Province of Moesia Inferior and destroyed many towns. Although Odessos remained intact during the Gothic raids, its economy declined. The town became prosperous again in the middle of the 4th century AD after Constantinople became the capital of the Roman Empire. Many people from Asia Minor, Syria and the Aegean Islands settled in Odessos, and a new fortification wall was built during the second half of the 4th century AD in order to protect the enlarged territory of the town. In that period, Odessos covered an area of about 32 ha. In the 4th century AD, Christianity rapidly spread in the town, although the pagan cults survived among the rural Thracian communities in its vicinity until the beginning of the 5th century AD. There was an Episcopate in the town and many Early Christian basilicas richly decorated with mosaics and paintings were built. In that period, Odessos became one of the most important commercial centers in the Eastern Roman Empire. In AD 536, during the reign of Emperor Justinian the Great, Odessos became the center of an extensive administrative district called Quaestura Exercitus, which incorporated several provinces. At the end of the 6th and the beginning of the 7th centuries AD, the Avars and the Slavs invaded the territory of the Eastern Roman Empire to the south of the Danube, and in AD 614, the inhabitants of Odessos abandoned the town and it was demolished. Later, during the Middle Ages, Odessos was under either Byzantine or Bulgarian rule.
Nikola Theodossiev, Sofia University