Sofia is one of the oldest European capitals. Its history dates back to ca 7th millennium BC. In 6th–5th century BC, there there had been a Neolithic settlement here.
In the 8th century BC, the Thracian tribe of the Serds settled around the thermal springs in the present-day downtown. Their settlement was called Serdonpolis. At the beginning of the 1st century AD, the Romans took the city and it became an administrative centre in the province of Thrace under the name of Serdica, and was later renamed Ulpia Serdica in honour of Emperor Trajan Marcus Ulpius.
At the end of the 3rd century AD, it became the capital of the province of Inner Dacia. The city suffered the incursions of Huns, Goths and other barbarian tribes in the 5th–6th century period.
In AD 809, it became part of the Bulgarian state and because of its central location received the name Sredets.
In the 1018–1194-period the city was included in the territory of the Byzantine Empire under the name of Triaditsa. Later, it was called Sofia. The oldest source mentioning the modern name of the city is a charter from 1382, presented by Tsar Ivan Shishman to the Dragalevtsi Monastery.
From the 14th century onwards, until 4 January 1878 the city was part of the Ottoman Empire. Sofia was liberated by the Russian army under the command of General Gurko.
On 3 April 1879, on a proposal of Professor Marin Drinov, Bulgarian historian, the Constitutive Assembly in Veliko Tarnovo designated the city for a capital of the Principality of Bulgaria.
Ancient Sofia - http://visitsofia.bg/catalog/en/Drevna_Sofia/index.html#/
Medieval Sofia - http://visitsofia.bg/catalog/en/Srednovekovna_SofiaEN/index.html#/
Modern-dey Sofia - http://visitsofia.bg/catalog/en/Moderna_SofiaEN/index.html#/