The closest is Zähringen Castle, 4 km north-east of the city centre; the most famous is Hohenzollern Castle — #58 of 8,133 worldwide — 91 km north-east. The oldest dated site in the circle is Fortified Church of St. Arbogast, founded 793.
The 100 km circle reaches into 3 countries — Switzerland (57), France (49), Germany (25).