Vienna's Imperial Palace is one of the biggest palace complexes in the world. The oldest parts date to the 13th century, with construction having continued right into the 20th century. The Imperial Palace was the residence and seat of government of the Habsburg emperors until 1918. Today, it is home to numerous museums with outstanding collections, the Spanish Riding School, a congress center, the seat of the Austrian Federal President as well as the historic Heldenplatz. https://www.wien.info/en/sightseeing/sights/imperial/hofburg-vienna