The closest is Citadel of Namur, 1 km south of the city centre; the most famous is Royal Palace of Brussels — #226 of 8,133 worldwide — 55 km north-west. The oldest dated site in the circle is Castle of Vêves, founded c. 650.
The 100 km circle reaches into 5 countries — Belgium (77), Netherlands (37), Luxembourg (5), Germany (2), France (1).