The earliest known settlements are those of the Celts dating back to the 3rd century BC. In the first decades BC, Transdanubia was conquered by the Romans who incorporated it into the Roman Empire as Pannonia, Aquincum. What is now Óbuda in the 3rd district, developed into the capital of the province Pannonia Inferior. This was the headquarters of the lcoa Roman governor and base to a significant military force which guarded the imperial frontier along the Danube River.
In the early fifth century, the Roman defence lines were swept away by the Goths and other tribes fleeing westwards from the Huns. During the flourishing period of the Hun empire (after AD 430), this crossing point over the Danube retained its significance.
The Hungarians appeared here around the end of the ninth century, establishing the seat of their prince near the same crossing of the Danube in the abandoned Roman settlement. They quickly recognized the geo-strategic significance of the place, and Obuda, the territory of the civilian city of Aquincum, became the first centre of Hungary. In the 16th century, most of Hungary, including Buda fell to the Turkish Ottoman Empire and was mostly destroyed in the 150 years of Turkish rule and in the war of liberation that ended it. From the late 17th century, Hungary was part of the Habsburg Empire, later called the Austro-Hungarian Monarchy, until the end of World War I.
In 1873, the formerly separate but interdependent towns, Buda, Pest and Óbuda were integrated into one administrative unit: Budapest. Due to concentration of capital and workforce and Budapest's pivotal position in the country's railway system, the city enjoyed a prosperity never seen before.
From the 1870s began the age of the Hungarian industrial revolution, the benefits of witch were mainly concentrated in Budapest. The city attracted the majority of newly-founded banks, business associations and industrial enterprises. This was the period when the face of today’s Budapest began to take shape.