The closest is Heeze Castle, 9 km south-east of the city centre; the most famous is Fort Eben-Emael (#234 of 9,035 worldwide) 72 km south. The oldest dated site in the circle is Kaiserpfalz Kaiserswerth, founded c. 700.
The 100 km circle reaches into 3 countries: Netherlands (134), Belgium (49), Germany (25).