The closest is Heeze Castle, 9 km south-east of the city centre; the most famous is Fort Eben-Emael — #233 of 8,133 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 (133), Belgium (35), Germany (16).