Serdica was originally a Thracian settlement, probably named after the Celtic Serdi tribe. For a short time in the 4th century B.C., the city was possessed by Philip of Macedon and his son Alexander the Great. Around 29 B.C., Sofia was conquered by the Romans and renamed Ulpia Serdica. It became a municipium (the center of an administrative region) during the reign of Trajan. The city expanded, as turrets, protective walls, public baths, administrative and cult buildings, a civic basilica and a large amphitheater were built. When Diocletian divided the province of Dacia into Dacia Ripensis (on the banks of the Danube) and Dacia Mediterranea, Serdica became the capital of Dacia Mediterranea. The city expanded for the next century and a half, which caused Constantine the Great to call it "my Rome." Serdica was destroyed by the Huns in 447, but was rebuilt by Justinian and surrounded with great fortress walls whose remnants can still be seen today. Although often destroyed by the Slavs, the town remained under Byzantine dominion until 809. Serdica is today Sofia, the capital of Bulgaria.