And the answer: Budapest.
In 1873, Budapest was joined together from 3 cities—Buda and Óbuda to the right, and Pest to the left of the Danube river—to become the capital city of Hungary. Budapest became the second-most eminent city in the Austro-Hungary empire after Vienna.
Now the capitol of Hungary (and all in one place!), Budapest originally existed as three separate cities along the Danube. Archaeological findings in the region suggest that the land has been used since the Stone Age, though the known history of Budapest begins with the foundation of the Roman town Aquincum, founded around 89 AD on the site of an old Celtic settlement near Óbuda. Territorial wars and changing ownership kept the influential cityscape in flux for hundreds of years.
At the end of the 17th century, though, the land came under control of the Habsburg monarchy. Suppression ensued, and although attempts to revolt were squashed, the attitude of the monarchy shifted in favor of the Hungarians following the turmoil of mid-17th century Europe. In 1867, the Habsburg monarchy was transformed into the Austro-Hungarian Empire, and Hungary was finally granted agency to rule themselves. In the booming economic development after the revolution and creation of the dual monarchy, Buda and Pest began developing into central European capitals. The independent cities of Pest, Buda and Óbuda were merged into one city called Budapest in 1873.
Today, Budapest is a global city with strengths in commerce, finance, media, art, fashion, research, technology, education and entertainment. As Hungary's financial center, Budapest was ranked as the second fastest-developing urban economy in Europe in 2014. The city houses the headquarters of several major businesses such as the European Institute of Innovation and Technology, the European Police College and the first foreign office of the China Investment Promotion Agency.
Learn more about the history of Budapest here.