Hagia Sophia is the second oldest church in Sofia, dating back to the 6th century. In the 14th century, the city of Sofia was named after Hagia Sophia. Hagia Sophia was built on the basis of several existing churches. In the 2nd century AD, it was a Roman theater. The main hall and two towers on the east side and a domed tower were all built in the middle of the 6th century during the reign of Byzantine King Justinian I. As one of the most precious early church buildings in southeastern Europe, the church floor has complex early Christian decorations or mosaics with floral and animal patterns, and there are tombs underground and around it that are older than the church was built. People believe that Hagia Sophia has been blessed by mysterious forces and has not been damaged by human invasion and natural disasters for centuries. It is an architectural representative of elegance, simplicity and symmetrical beauty at that time.