The Church St. Sofia, known as The Old Bishopric, is situated on the place supposed to be the old Centrum of the settlement. It is built in 5th C. AD and is a three-nave basilica with a semicircular apse, a three-part narthex and an atrium (an inner yard of 20m width and 26m length). The central nave is separated from the two side aisles with orthogonal stone columns and brick arches. Above there is a second arcade. There are three arch windows over the apse at the East Side and a two slopes roof structure, not preserved. The apse possesses a synthronon, which can be seen today. Inside the church is plastered up and covered with frescoes. The whole floor was covered with a mosaic of small multicolored stones.
There were two periods in its construction. The basilica was reconstructed in the 7th century and rebuilt in the beginning of the 9th century. In the Middle Ages it served as a cathedral of the bishop with its residence in Nessebar. In 1257 it was looted by the Venetians, and many religious relics were taken to the church of San Salvatore in Venice.
At the end of the 14th and the beginning of the 15th century, Mataisa Kantakuzina-Palaeologus was buried in St. Sophia, and the inscription on her tombstone is still preserved. The Church of St Sofia was used until the end of 17th C. and after that it was abandoned.