UNESCO's World Heritage Site, the opulent summer palace of the Habsburgs, boasts 1441 rooms. Forty of them are open to the public for exploration, including the private apartments of Empress Sisi and Emperor Franz Joseph. The Imperial Tour covers 22 rooms, and the Grand Tour covers 18 more rooms with 18th-century interiors from the time of Maria Theresia. Both tours are mandatory, but the Grand Tour can be done with a tour guide or with the provided audio guide (or app).