The closest is Hegi Castle, 4 km east of the city centre; the most famous is Hohenzollern Castle — #58 of 8,133 worldwide — 93 km north. The oldest dated site in the circle is Heuneburg, founded c. 300.
The 100 km circle reaches into 5 countries — Switzerland (86), Germany (24), Liechtenstein (4), Austria (4), France (1).