The closest is Electoral Palace (Trier), 1 km south-east of the city centre; the most famous is Eltz Castle (#74 of 9,035 worldwide) 71 km north-east. The oldest dated site in the circle is Celtic circular wall of Otzenhausen, founded c. 350 BC.
The 100 km circle reaches into 4 countries: Germany (97), Luxembourg (49), France (5), Belgium (4).