The oldest and best preserved monument of architecture since Roman times in Sofia is the rotunda church St. George. It was built in the beginning of the 4th century AD and, was considered, during the reign of the roman emperor Constantine the Great (reigned 306 – 337) and the flourishing of the ancient town Serdika (the old name of Sofia). It is believed that the rotunda is the oldest building in Sofia. The temple is situated in the courtyard of the Government Office, just a couple of meters of the remains of the ancient fortress Serdika. It is a cylindrical domed structure, built on a square base. Its width is about 9.5 meters in diameter and its height is about 14 meters. The altar room has a square form and four symmetrically placed niches. The main entrance is at the western wall. Originally the building was used for public purposes. After the recognition of Christianity as a religion in the Roman Empire, the rotunda became a baptistery (a building for conversion to Christianity), due to the many conversions, following the authorization of this religion. http://bulgariatravel.org/en/object/234/sofia