The city of Budapest was officially created on 17 November 1873 from a merger of the three neighboring cities of Pest, Buda and Óbuda. Smaller towns on the outskirts of the original city were amalgamated into Greater Budapest in 1950. The origins of Budapest can be traced to Celts who occupied the plains of Hungary in the 4th century BC. The area was later conquered by the Roman Empire, which established the fortress and town of Aquincum on the site of today's Budapest around AD 100. The Romans were expelled in the 5th century by the Huns, who were challenged by various tribes during the next several centuries. The Hungarian conquest of the Carpathian Basin started at the end of the 9th century, and the Kingdom of Hungary was established at the end of the 11th century.