Claim: Budapest was the co-capital of the Austro-Hungarian Empire.

Budapest -LRB- -LSB- ˈbudɒpɛʃt -RSB- -RRB- is the capital and most populous city of Hungary and one of the largest cities in the European Union . It has an area of 525 km2 and a population of about 1.8 million within the administrative limits in 2016 . Budapest is both a city and county , and forms the centre of the Budapest Metropolitan Area , which has an area of 7,626 km2 and a population of 3.3 million , comprising 33 percent of the population of Hungary . Budapest became a single city occupying both banks of the Danube river with the unification of Buda and Óbuda on the west bank , with Pest on the east bank on November 17 , 1873 .   The history of Budapest began with Aquincum , originally a Celtic settlement that became the Roman capital of Lower Pannonia . Hungarians arrived in the territory in the 9th century . Their first settlement was pillaged by the Mongols in 1241 -- 1242 . The re-established town became one of the centres of Renaissance humanist culture by the 15th century . Following the Battle of Mohács and nearly 150 years of Ottoman rule , the region entered a new age of prosperity , and Budapest became a global city after its unification in 1873 . It also became the co-capital of the Austro-Hungarian Empire , a great power that dissolved in 1918 , following World War I. Budapest was the focal point of the Hungarian Revolution of 1848 , the Hungarian Republic of Councils in 1919 , the Battle of Budapest in 1945 , and the Hungarian Revolution of 1956 .   Budapest is a Beta + global city , with strengths in arts , commerce , design , education , entertainment , fashion , finance , healthcare , media , services , research , and tourism . Its business district hosts the Budapest Stock Exchange and the headquarters of the largest national and international banks and companies . Budapest is a leading R&D and financial centre in Central and Eastern Europe , the city has a gross metropolitan product more than $ 100 billion in 2015 , thereby the city is among the Top 100 GDP performing cities in the world  and making it one of the largest regional economies in the European Union . It is the highest ranked Central and Eastern European city on Innovation Cities Top 100 index . Budapest attracts 4.4 million international tourists per year , making it the 25th most popular city in the world and the 6th in Europe .   Among Budapest 's important museums and cultural institutions , the most visited art museum is the Museum of Fine Arts , as well as the Hungarian National Gallery , which is noted for one of the largest collections of all periods of European art and comprises more than 100,000 pieces . The central area of the city along the Danube River is classified as a UNESCO World Heritage Site and has many notable monuments , including the Hungarian Parliament , Buda Castle , Fisherman 's Bastion , Gresham Palace , Széchenyi Chain Bridge , Matthias Church and the Liberty Statue . Further famous landmarks include Andrássy Avenue , St. Stephen 's Basilica , Heroes ' Square , the Great Market Hall , the Nyugati Railway Station built by the Eiffel Company of Paris in 1877 and the second-oldest metro line in the world , the Millennium Underground Railway . Budapest is home to numerous museums , galleries , libraries , sporting events and other cultural institutions , including the Hungarian National Museum , House of Terror , Franz Liszt Academy of Music , Hungarian State Opera House , National Széchényi Library and Hungarian Academy of Sciences .   Budapest is cited as one of the most beautiful cities in Europe , ranked as the most liveable Central and Eastern European city on EIU 's quality of life index , ranked as `` the world 's second best city '' by Condé Nast Traveler , and `` Europe 's 7th most idyllic place to live '' by Forbes . It has around 80 geothermal springs , the world 's largest thermal water cave system , second largest synagogue , and third largest Parliament building . Budapest is home to the headquarters of the European Institute of Innovation and Technology , the European Police College and the first foreign office of the China Investment Promotion Agency . Over 40 colleges and universities are located in Budapest , including the Eötvös Loránd University , Central European University and Budapest University of Technology and Economics .