The ruined castle of Franchimont stands on a ridge about 7 kilometres from Spa, near the village of Theux. What remains is substantial: the outline of the towers, the walls, and enough of the interior to understand the layout of a 12th-century fortified complex. The view from the castle ramparts over the wooded Ardennes valleys is excellent.
The castle is most famous for the story of the 600 men of Franchimont — local volunteers who attacked the Burgundian army besieging Liège in 1468, hoping to kill Charles the Bold and end the siege. They came close but failed; all 600 died and Charles had the castle demolished in retribution. The ruins are freely accessible and a good destination for a half-day excursion from Spa.