Celle’s wedding-cake Schloss was built in 1292 by Otto Der Strenge (Otto the Strict) as a town fortification; in 1378 it was expanded and turned into a residence. Today it houses administrative offices, a theatre and the ResidenzMuseum, which lets you walk through various former state apartments and rooms to see clothing, weaponry and other historical items on display. There's some English signage throughout.
From May to October, one-hour guided tours in German (adult/child €9/free) depart at 11am, 1pm and 3pm from Tuesday to Friday and Sunday, and hourly from 11am to 3pm on Saturday. (From November to April there are fewer; see the website for more information.) These tours take you into sections of the palace you can’t otherwise visit, such as the Renaissance Schlosskapelle (Palace Chapel), the 19th-century Schlossküche (Palace Kitchen) and – rehearsals permitting – the Baroque Schlosstheater (Palace Theatre). You can also purchase a great-value combination ticket that allows you to visit the Schloss, the Bomann-Museum and the Kunstmuseum; it's good for two consecutive days.
The last duke in residence here was Georg Wilhelm (1624–1705), while the last royal was Queen Caroline-Mathilde of Denmark, who was exiled here after having an affair with a court physician and died in 1775 from scarlet fever at the tender age of 23. You'll see some of her clothing and personal effects in the museum.
Don't miss a stroll around the lovely grounds, where weeping willows stand guard over a meandering circular moat populated by ducks and geese.