The closest is Palace of Aachen, 1 km south of the city centre; the most famous is Fort Eben-Emael — #233 of 8,133 worldwide — 28 km west. The oldest dated site in the circle is Castle of Vêves, founded c. 650.
The 100 km circle reaches into 4 countries — Netherlands (54), Germany (47), Belgium (39), Luxembourg (9).