information:classification, structural:summarization
Prehistory and antiquity

The area has a history of nearly 7,000 years, with the great attraction of the hot water springs that still flow abundantly in the centre of the city. The neolithic village in Slatina dating to the 5th–6th millennium BC is documented. Another neolithic settlement was founded in the 3rd–4th millennium BC near the site of the modern National Art Gallery, which has been the traditional centre of the city ever since.

The earliest tribes who settled were the Thracian Tilataei. In the 500s BC, the area became part of a Thracian state union, the Odrysian kingdom from another Thracian tribe the Odrysses.

In 339 BC Philip II of Macedon destroyed and ravaged the town for the first time.

The Celtic tribe Serdi gave their name to the city. The earliest mention of the city comes from an Athenian inscription from the 1st century BC, attesting Astiu ton Serdon, i.e. city of the Serdi. According to the inscription and to the writings of Dio Cassius, the Roman general Crassus subdued the Serdi and behanded the captives.

Dio Cassius, Pliny the Elder and Ptolemy say that in 27–29 BC Crassus attacked the region "Segetike", which is assumed to be Serdica, or the city of the Serdi. The ancient city is located between TZUM, Sheraton Hotel and the Presidency. It gradually became the most important Roman city of the region. It became a municipium during the reign of Emperor Trajan (98–117). Serdica expanded, as turrets, protective walls, public baths, administrative and cult buildings, a civic basilica, an amphitheatre, a circus, the City council (Boulé), a large forum, a big circus (theatre), etc. were built. Serdica was a significant city on the Roman road Via Militaris, connecting Singidunum and Byzantium. In the 3rd century, it became the capital of Dacia Aureliana, and when Emperor Diocletian divided the province of Dacia Aureliana into Dacia Ripensis (at the banks of the Danube) and Dacia Mediterranea, Serdica became the capital of the latter. Serdica's citizens of Thracian descent were referred to as Illyrians probably because it was at some time the capital of Eastern Illyria (Second Illyria).

Roman emperors Aurelian (215–275) and Galerius (260–311) were born in Serdica.

The city expanded and became a significant political and economical centre, more so as it became one of the first Roman cities where Christianity was recognised as an official religion (under Galerius). The Edict of Toleration by Galerius was issued in 311 in Serdica by the Roman emperor Galerius, officially ending the Diocletianic persecution of Christianity. The Edict implicitly granted Christianity the status of " religio licita ", a worship recognised and accepted by the Roman Empire. It was the first edict legalising Christianity, preceding the Edict of Milan by two years.

Serdica was the capital of the Diocese of Dacia (337–602).

For Constantine the Great it was 'Sardica mea Roma est' (Serdica is my Rome). He considered making Serdica the capital of the Byzantine Empire instead of Constantinople. which was already not dissimilar to a tetrarchic capital of the Roman Empire. In 343 AD, the Council of Sardica was held in the city, in a church located where the current 6th century Church of Saint Sophia was later built.

The city was destroyed in the 447 invasion of the Huns and laid in ruins for a century It was rebuilt by Byzantine Emperor Justinian I. During the reign of Justinian it flourished, being surrounded with great fortress walls whose remnants can still be seen today.