The closest is Electoral Palace (Trier), 1 km south-east of the city centre; the most famous is Eltz Castle — #75 of 8,133 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 (81), Luxembourg (46), Belgium (3), France (2).