There has been some kind of fortification at Bouillon for over a thousand years, guarding the Semois valley, which is an obvious route for invasions of France. The Duchy of Bouillon was owned by the Dukes of Ardenne until 1096, when it passed into the hands of the Prince-bishops of Liège. In the 16th century, the title of Duke of Bouillon was usurped by the La Marck family. In 1591, Henry de la Tour d'Auvergne, father of Marshal Turenne, inherited the title but did not own the castle or the Duchy. In 1676 Louis XIV's troops captured the castle of a 20-day siege. The town, castle and Duchy were handed to the La Tour d'Avergne family in 1678.
After Bouillon had been captured by the French Louis XIV sent his military engineer Vauban to improve the defences there. Seeing the importance of Bouillon to the defence of France's northern frontier, Vauban referred to the fortress as the "key to the Ardennes". Lying in a loop of the river Semois, Bouillon presented a similar situation to Besançon, but on a smaller scale. The heights at the narrow neck of the loop were already protected by the existing castle, although the defences dated from medieval times.
As at Besançon, Vauban used roofed tower bastions where the river prohibited the construction of a full bastioned trace. He built nine tower bastions, connected by curtain walls to enclose the town of Bouillon, with the walls running up the steep slopes to the castle at the neck of the loop. The river in front of these fortifications acted as a large ditch. The tower bastions had wooden roofs made with thick timbers to shelter the defenders from any shots coming from the surrounding hills that dominated the town.
In addition to fortifying the town, Vauban made various improvements to the castle to adapt it for artillery. The castle has three successive ditches that were hewn out of the rock, each spanned by a drawbridge. These create three sections to the fortress, the first two being the smallest, effectively large fortified gatehouses. The highest section was the biggest and the strongest. On the south-east side, which was closest to the river, Vauban added a long, high loopholed wall overlooking the river and road below. The loopholes were unusual in allowing each defender only three directions fire (he could fire either 45° to left, straight ahead, or 45° to the right). This restricted the field of fire, but gave greater protection to the defenders on this wall. Vauban may have decided that the wall was too inaccessible to be assaulted, so the price of this protection did not weaken the place. He also added a semi-circular bastion to flank this wall with artillery. On the other side of the castle, Vauban built a circular powder magazine, which mounted guns on its roof.
The highest point of the fortress, which is at the south-west end, is known as the Tour d'Autriche (Austrian Tower), a large, long tower that was converted to mount artillery by George of Austria (Prince-bishop of Liège) in 1551. This tower has embrasures on all sides. Originally the tower was roofed to shelter the gunners from the weather and enemy fire. Below the Austrian Tower are two gun batteries, which can command the whole valley between them. Beyond the Austrian Tower lies the hill of Montagne de Bémont. This high ground is in fact higher than the castle, but several factors meant that this was not an overwhelming weakness for the castle. Firstly, the ridge of the Montagne de Bémont that leads to the castle is sharp-edged and rocky, which would make it impossible for an attacker to dig batteries there and would hinder an infantry attack. Secondly, a man-made cut was made in medieval times to separate the castle from the ridge. This cut was deepened on Vauban's orders, making an assault on this end of the castle very difficult.
Below the higher sections of the fortress there are several walled platforms for musketry defence that were built into the side of the hill. Often reached by passages carved out of the rock itself, these plaftorms enabled the garrison to cover the slopes on the sides of the castle and protect them from attack. Most of the platforms themselves are completely inaccessible from the outside, so they were never at risk of being assaulted.
The work of Vauban (and previous engineers working for the Bishops of Liège) converted the castle into an artillery fortress, but it still retained its medieval character. This was mainly due to the nature of the terrain. The strength of the castle lay in its position, dominating the valley on both sides and the town to the north-east. In many places there was no need for the angular bastions, outworks and covered way that 17th century military fortifications normally have - the cliffs were simply unassailable and modern works would not have made any difference to the defence.
The castle of Bouillon is in very good condition and can be visited for a small fee. There is an audio guide available and a good guide-book in various languages. The visitor is free to explore the whole castle, including the tunnels beneath the Austrian Tower that give access to the musketry platforms in the side of the hill. The castle is also home to some birds of prey, which are flown several times a day by their keeper from an arena in the centre of the upper castle.
Only three of the seven original tower bastions defending the town have survived, one of which is just beneath the castle. The other two of have been modified to take the road that runs around the north of the town. The walls between the tower bastions have, for the most part, long since disappeared. The Musée Ducal in the town has on display a copy of the relief map of Bouillon's fortifications originally made in 1699. There is no railway station at Bouillon, but it is easily accessible by road, on the N89 a short distance from Charleville.