The first settlement on the territory of Budapest was built by Celts before 1 AD. It was later occupied by the Romans. The Roman settlement – Aquincum – became the main city of Pannonia Inferior in 106 AD. At first it was a military settlement, and gradually the city rose around it, making it the focal point of the city's commercial life. Today this area corresponds to the Óbuda district within Budapest. The Romans constructed roads, amphitheaters, baths and houses with heated floors in this fortified military camp. The Roman city of Aquincum is the best-conserved of the Roman sites in Hungary. The archaeological site was turned into a museum with inside and open-air sections.