The Habsburgs' opulent summer palace is now a Unesco World Heritage site. Of its 1441 rooms, 40 are open to the public; the Imperial Tour takes you into 22 of these, including the private apartments of Franz Joseph and Sisi. The Grand Tour continues to the other 18 rooms and includes the 18th-century interiors from the time of Maria Theresia. These mandatory tours are done with a free audio guide (or app); the Grand Tour can also be done with a tour guide.
Due to the palace's popularity, tickets are stamped with a departure time and there may be a gap of an hour or more, so buy your ticket straight away and then explore the gardens, or book and buy for a specific time online.