In Sofia there are 607,473 dwellings and 101,696 buildings. According to modern records 39,551 dwellings were constructed until 1949, 119,943 between 1950 and 1969, 287,191 between 1970 and 1989, 57,916 in the 90s and 102,623 between 2000 and 2011. Until 1949, 13,114 buildings were constructed and between 10,000-20,000 in each following decade. Sofias architecture combines a wide range of architectural styles, some of which are aesthetically incompatible. These vary from Christian Roman architecture and medieval Bulgar fortresses to Neoclassicism and prefabricated Socialist-era apartment blocks. A number of ancient Roman, Byzantine and medieval Bulgarian buildings are preserved in the centre of the city. These include the 4th century Church of St. George, Sofia, the walls of the Serdica fortress and the partially preserved Amphitheatre of Serdica.
Question: What buildings are preserved in the center of the cities?
Answer: ancient Roman | medieval Bulgarian | Byzantine