The closest is Citadel of Namur, 1 km south of the city centre; the most famous is Royal Palace of Brussels (#225 of 9,035 worldwide) 55 km north-west. The oldest dated site in the circle is Castle of Vêves, founded c. 675.
The 100 km circle reaches into 5 countries: Belgium (113), Netherlands (38), Luxembourg (5), Germany (3), France (2).