The closest is Düsseldorfer Schloss, 1 km north-west of the city centre; the most famous is Fort Eben-Emael — #233 of 8,133 worldwide — 90 km south-west. The oldest dated site in the circle is Kaiserpfalz Kaiserswerth, founded c. 700.
The 100 km circle reaches into 3 countries — Netherlands (68), Germany (59), Belgium (8).