The closest is Hegi Castle, 4 km east of the city centre; the most famous is Hohenzollern Castle (#58 of 9,035 worldwide) 93 km north. The oldest dated site in the circle is Heuneburg, founded c. 600 BC.
The 100 km circle reaches into 5 countries: Switzerland (95), Germany (31), Austria (5), Liechtenstein (4), France (1).