The Budapest Metropolitan Area is home to 3.3 million people. The city covers an area of 525 square kilometres (202.7 sq mi). Budapest became a single city occupying both banks of the river Danube with the unification of Buda and Óbuda óbuda on the west, bank With pest on the east bank on 17 november. 1873 the history Of budapest began With, aquincum originally A celtic settlement that became The roman capital Of Lower. pannonia