The position of Sofia today is known as a crossing road point of great importance thousands of years ago. The shortest way between Europe and Asia is the natural northwest - southeast diagonal passing through The Balkan Peninsula and crossing The Sofia plain. The other important road descends from North at the same place and continues to The Egey Sea. At this crossroad live flows for at least 8000 thousand years, and the city itself is founded by the Thracian tribe Serdi under the name Serdonpolis and it has existed for more than 2000 thousand years. The city of the Serdi people was conquered by the Romans in the beginning of 1st A.D.
During the Roman era ancient Serdica is one of the significant cities in the interior of The Balkan Peninsula and exists four centuries within the Roman Empire, and after that - four more under the rule of Byzantium. Marcus Ulpius Traianus(98-117) from whom the city gets its name Ulpia initiated the development of public services and construction works of the city. Serdica became capital of the new made province Inner Dacia (Dacia Mediterranea) during the last quarter of the 3rd century. In the beginning of the 4th century at the time of Constantine the Great, or Constantin I Serdica was his residence for a while. It is even said that he called it "My Rome".
A better picture of the urbanization, the architecture and the history of Ulpia Serdica can be drawn from the excavations and the research so far in different parts of the city included within the inner fortified wall. Serdica had very good water - main system and system of sewers, carefully maintained. Eight miles long water - main brought water from Vitosha Mountain capping in today's district Boyana. North of The Central Department Store has been found parts of the spacious ancient baths of Serdica. Around the forum were concentrated the more important public buildings.
Serdica has three walls built during different periods. The excavations show that the city has been destroyed many times by Huns, Goths, Bulgarians and Ottomans. From the monumental buildings from that time is preserved in good shape only the cult building (martyrion) from the 4th century, known as Saint George Rotunda.
The oldest found walls, made from stone and mud date as far back as the beginning of The Roman dominion in Serdica and Thracia as a whole (1st A.D.). Other walls can be referred to the 2nd and the beginning of 3rd century. Subsequent are the buildings with mixed masonry (opus mixtum) with belts each by three rows of brick. This is the time of the Huns attacks - the middle or the end of the 3rd century and the beginning of the 4th. Immediate after that follows the period characterized with masonry but with 4 rows of bricks in a belt.
The revealing of Serdica begins after the bomb attack in 1944 which necessitates the reconstruction and the rebuilt of the stricken center. For the new planned buildings - hotel "Balkan" (today "Sheraton"), the present National Assembly building, TsUM and The Council of Ministers - a large space is excavated. This is when; between hotel "Sheraton" and The Office of the President of Bulgaria are found remains of a Thracian settlement on top of which lay the foundations of the roman city Ulpia Serdica.