Sofia (Bulgarian: София, ) is the capital and largest city of Bulgaria and the 15th largest city in the European Union with a population of 1.3 million people. It is located in western Bulgaria, at the foot of Mount Vitosha and approximately at the centre of the Balkan Peninsula.
Prehistoric settlements were excavated in the centre of the present city, near the royal palace, as well as in outer districts Slatina and Obelya. The well-preserved town walls (especially their substructures) date back before the 7th century BC, when Thracians established their city around a mineral spring, which exists to the present day. Sofia has had several names in the different periods of its existence. Its ancient name, Serdika or Serdica, derives from the local Celtic tribe of the serdi who inhabited the region since the 1st century BC. Serdica was a Roman capital during the tetrarchic system of government. During the Middle Ages, it was one of the major commercial centres of the Bulgarian Empire, along with Tarnovo. Sofia's population remained small until 1879, when it was declared a capital of the Principality of Bulgaria after the Liberation of Bulgaria from Ottoman rule.
Sofia has been ranked by the Globalization and World Cities Research Network as a beta- world city. Many of the major universities, cultural institutions, and businesses of Bulgaria are concentrated in Sofia.