Bouillon Castle is one of the most impressive medieval fortifications in Belgium, perched on a rocky spur above the tight bend of the Semois River with the town spread below on both sides. The castle dates to at least the 10th century, with the core of the current structure built in the 11th and 12th centuries. Three drawbridges, a dungeon, towers at different levels, and subterranean passages are all accessible.
The views from the towers over the river and forested hills are the real reward. The castle is well maintained and the audioguide is unusually informative, covering both the medieval history and the later role of the fortress in the conflicts between France and the Habsburg Netherlands. Allow at least 90 minutes. From the top the coiled loops of the Semois are unmistakeable — you understand immediately why this ridge was worth defending.