The city of Luxembourg lies high in the Ardennes at the confluence of the Alzette and Pétrusse rivers. Although these rivers are small, they flow at the bottom of deep valleys, providing the city with natural defences on three sides. This strong natural position was a key factor in the development of one of the most powerful fortresses in Europe.

The Romans built a watchtower here, guarding the road between Rheims and Trier, but it was not until the middle ages that a town grew up on the site. This was largely due to Count Siegfried, who built a castle on the Bock promontory in the 10th century. The Bock is an extremely strong natural feature, being protected by steep cliffs on every side except for a narrow strip of land in the east, which connects it with the plateau on which the town is situated.

The town of Luxembourg grew up in the shelter of Siegfried's castle and in the 12th century a new circuit of walls had to be constructed to take in the enlarged town.

As the city expanded, it spread along the banks of the Alzette to the north and onto the Rham Plateau, a tongue of high ground to the south of the Bock on the far side of the Alzette. In order to protect these new suburbs, another set of walls was built in the 15th century.

Luxembourg was captured in a surprise night-attack by the Burgundians in 1443, passing to the Habsburgs in 1477 along with the rest of the Netherlands. The province did not revolt against Spanish rule along with the other Netherlands during the 80 Years War'but its fortifications were given some attention in the 16th century.

The first adaptation of the medieval walls for artillery came in the form of a single arrow headed bastion'built at an angle of the walls in the west. Although minimal, this work meant that the whole of the western side of the defences could be swept with cannon fire.

The city changed hands a number of times in the mid-16th century wars between France and Spain, resulting in further modifications being undertaken by the Spanish throughout the second half of the 16th century and into the 17th century. This work focussed on the defences in the west, where the were no cliffs to deter an attacker.

The original central bastion'was given two neighbours that anchored the north and south ends of the western defences and a demi-lune'was constructed between the central and southern bastions. The new bastions increased the flanking capability along the western walls.

The Bock was also strengthened at this point, as were the defences on the south side of the town. A significant development was a hornwork'on the south side of the Pétrusse, which prevented an enemy from bombarding the Plateau du Saint-Esprit (the southern arm of the town).

In 1635, France declared war on Spain and in the 1640s they made successful advances into the Spanish Netherlands. Concerns over the safety of Luxembourg resulted in a large-scale project to strengthen the defences. The work was supervised by a Swiss engineer called Isaac von Treybach.

Treybach had the western defences revetted in stone and added demi-lunes, counterguards'in front of the bastions and a covered way. This front, with its arrow headed bastions and counterguards, bears a resemblance to the fortifications of Valletta, the renowned fortress on Malta that built in the second half of the 16th century. It is quite likely that Treybach was influenced by the works on Malta, which were already famous throughout Europe.

The cliff-top walls around the rest of the town were also strengthened, especially in the south with the construction of the large Beck Bastion and various other works dominating the Pétrusse valley. The cliffs on the north side of the town are steeper, so less work was carried out there.

The defences lining the Pétrusse valley were made even more formidable by casemates'carved out of the rock beneath the walls. The guns sited in these underground chambers would have been very hard to attack because they fired through small openings in the rock.

The Bock was also revamped with three separate forts, the Large Bock, Middle Bock and Small Bock (from west to east). These three elements were separated by cuts in the rock and were linked by bridges. By this time little remained of the medieval castle.

The hornwork on the far side of the rivers was probably an earthwork, and Treybach did not restore it. The only work that was built on the opposite bank of the rivers to the town was a small demi-lune at the rear of the Rham Plateau.

In 1659 the Treaty of the Pyrenees'resulted in some territory in the province of Luxembourg going to France, including the important fortress of Thionville. This put Luxembourg on the front line, which led to more reinforcement work in the 1670s.

Between 1671 to 1684, a strong demi-lune called the Ravelin du Pâté was built next to the Beck Bastion and a line of 4 redoubts'was added in the west to give the defences there an extra layer. In 1683 the French king Louis XIV'claimed that Luxembourg belonged to France and demanded that Spain give it up.

When the Spanish failed to comply, Louis sent an army to Luxembourg in 1684. There followed a hard-fought siege lasting over a month. The attacks were directed by Vauban, the famous French engineer. The main thrust was aimed at the north end of the western defences.

The French advance was slow due to the various redoubts and outworks that had to be overcome, and was also hindered by the garrison's countermines. Vauban also made attacks in the east, taking advantage of the undefended high ground on the right bank of the Alzette.

Finally the French clawed their way to the walls and made a breach, forcing the garrison to surrender. By the end of the siege the ground in front of the western sector was riddled with trenches, many of the outworks were damaged and the Bock had been flattened by the French guns.

The Treaty of Ratisbon, signed later that year, put an end to the war between France and Spain. The peace allowed Vauban to spend time repairing and strengthening the fortifications he had recently been attacking. Over the next few years the French spent vast amounts of money transforming Luxembourg into one of Europe's most powerful fortresses.

The western defences had already proved their strength in the siege, so Vauban did not need to make many improvements there. One thing he did do was to build 3 new redoubts in the gaps between the existing 4, strengthening the outer line of defence in the west.

Having placed batteries on the heights on the far bank of the Alzette during the siege, Vauban was well aware of this danger and he ensured that they were well fortified.

Firstly, there was Fort Bas-Grünewald (or Fort Niedergrünewald), a work with three bastions and two demi-lunes protecting the heights above the Pfaffenthal suburb. It was from here that French batteries had enfiladed the main attack in 1684.

Secondly, on another hill just to the south of the heights of Pfaffenthal and seperated from them by a valley, there was Fort Haut-Grünewald (or Fort Obergrünewald).

This fort was essentially a hornwork'with a demi-lune. Both the Grünewald forts were unusual in that they faced uphill. Usually fortifications were placed so that the ground sloped down in front of them, so that they could not be overlooked by an attacker.

Vauban considered this a price worth paying for the ability to defend the right bank of the Alzette, although it was a concern to future engineers, leading to the construction of successive works in front of both forts in an attempt to offset this disadvantage.

At any rate the ground does not slope steeply upwards, but there are steep cliffs facing the valley in their rear. Vauban left the rear of both forts open, so that if they did fall into the hands of an enemy, they would be vulnerable to fire from the town's guns.

The two forts were not isolated, but were integrated into the rest of the defences by communication walls. Fort Niedergrünewald was linked to the western fortifications of the town by a wall running across the Alzette valley.

This wall had two gates, the Eicher Gate on the left bank of the river and the Siechen Gate on the right bank. Vauban built them as defensive works, not merely ornamental gatehouses. They were tall stone towers with loopholes, strong enough to deter an enemy from sending troops along the valley to take the Grünewald forts from behind. These gates, which became known as the Vauban Towers, had no artillery but were designed for infantry defence with muskets.

From the Berlaimont Redoubt at the northern end of the western defences, a loopholed wall ran down the cliff to the Eicher Gate. From here the wall went straight across the river, where it crossed a bridge equipped with sluice gates, to the Siechen Gate and up the hill to Fort Niedergrünewald.

For almost all of its length, the wall was fronted by a deep dry ditch'and could be defended by musketeers firing through the loopholes, presenting a formidable obstacle to an attacker. The lack of artillery defence for this communication wall was made up for by the guns in Fort Niedergrünewald at the east end and the Berlaimont Redoubt at the west end. These two positions dominated the whole valley and flanked the length of the wall.

Between the two Grünewald forts, there is a small valley. Although it is much narrower than the Alzette valley, Vauban still had a communication wall built across it to link the two forts.

Halfway along this wall was the Grünewald Gate, where the Roman road to Trier left the town. Although smaller than the other two gates, the principle was the same - a solid stone tower built for infantry defence and supported by the artillery in the Grünewald forts to either side. The final section of the communication wall recrossed the Alzette, linking Fort Obergrünewald with the Bock. It was covered by the guns in the right flank of Fort Obergrünewald and in the Middle Bock.

The Bock itself, which had been largely destroyed during the siege, was rebuilt to its previous form by Vauban. Its guns dominated the Alzette valley to the north and the Rham Plateau to the south. A small redoubt was built to the east to protect the approaches to the Bock.

At the Rham Plateau, Vauban built a demi-lune in front of the medieval walls and an advanced lunette called the Rumigny Redoubt. The plateau was also the site of some new barracks that the French constructed to house the larger garrison that was needed to man the new fortifications.

At the confluence of the two rivers, inside the town, the French built a citadel, perhaps partly to control the population, but also to act as a final refuge. The Citadelle du Saint-Esprit was situated in one of the most unassailable parts of Luxembourg's fortifications. On the west, south and east sides it was protected by the cliffs of the steep-sided Pétrusse and Alzette valleys.

The only part not protected by cliffs was north front, which faced into the town. Vauban remodelled the existing walls above the cliffs, giving an extra layer to the defences in places. For the northern front he built two demi-bastions'and a demi-lune, with a deep ditch.

Since the citadel could only be attacked from the town, a besieger would be forced to fight his way through the western fortifications and then to attack the citadel.

To prevent the citadel from being bombarded from the adjacent heights, the old Spanish hornwork on the south side of the valley was rebuilt, this time in stone. It was not the only work to be built on the south bank of the Pétrusse; opposite the Beck Bastion was a demi-lune known as Fort Bourbon, and between this demi-lune and the hornwork was a small lunette, the Bourbon Redoubt. These three works were linked by a covered way.

The hornwork was linked to the Rham Plateau and to the citadel by communication walls running across the valleys, but the other two works had no direct links with the town.

Vauban also remodelled the Beck Bastion, making it significantly higher so that the defences were layered, with the lower works being dominated by the walls to their rear. The guns in these defences covered Fort Bourbon and the Bourbon Redoubt, which were both open at the rear.

Although the French spent huge sums of money strengthening the fortress of Luxembourg, it was only in their hands for 13 years. In 1697 the Treaty of Rijswijk'returned Luxembourg to Spanish control. In 1701 it received a French garrison again, but the fortress was not besieged during the War of the Spanish Succession.

In 1713 the Spanish Netherlands passed to the Austrians, who carried out more work to enlarge and strengthen the fortress of Luxembourg. Firstly, extra outworks were added to the west, reinforcing the line of redoubts.

Vauban's Bourbon Redoubt and Fort Bourbon were supplemented by Fort Elisabeth and various outworks. Fort Bourbon was linked to the town by a tunnel that ran across the valley through the Bourbon Sluice.

More redoubts were built to defend the approaches to the Rham Plateau and forts Niedergrünewald and Obergrünewald. The rear of these two forts, which Vauban had left open, was closed up by the Austrians.

They built a redoubt in the rear of each fort that was used as a guard house. Vauban had left both forts open towards the town, so as to avoid them providing shelter for an attacker, but it seems the Austrian engineers preferred enclosed works.

The most significant work undertaken by the Austrians, besides the multiplication of the outworks, was the construction of the Bock Casemates. These underground chambers carved into the Bock had narrow openings for artillery.

The guns sited there could cover the entire Pétrusse valley to the north as far as the Vauban Towers and the Rham Plateau to the south. Their positions in the rock made them almost invulnerable to enemy fire.

In the underground chambers and the superstructure on top of the rock, there were around 50 guns, a huge amount of concentrated artillery. There were also tunnels linking the three sections of the Bock with each other and the town, in case the bridges were destroyed.

These casemates significantly strengthened the fortress, effectively closing off the approaches to the east of the town.

In 1732 Fort Thüngen was built in front of Fort Obergrünewald, incorperating the Redoute du Parc, a small work built in Vauban's time. Fort Thüngen was named after the Austrian commander at Luxembourg, Adam Sigmund Freiherr von Thüngen.

In 1795 the defences were finally put to the test when a French Revolutionary army laid siege to Luxembourg. The Austrian garrison surrendered after a siege of over 7 months, due to lack of supplies. This siege earned Luxembourg the name Gibraltar of the North, after the strong Mediterranean fortress built on a rock on the Spanish coast.

In 1815 after the Napoleonic Wars, the Grand Duchy of Luxembourg was created and the fortress was garrisoned by the Prussians. During the Prussian period, some of the outlying works were remodelled, including Fort Thüngen, which was given three iconic towers, topped with acorns. In addition, as the range of artillery increased, new forts were built far in advance of the earlier fortifications, which had been designed to defend the town against shorter-ranged guns.

In 1867 France sought to strengthen its eastern frontier by purchasing the Duchy of Luxembourg from the Netherlands, thereby gaining control of the fortress. The Prussians opposed this and refused to withdraw their own troops from Luxembourg. Neither side was willing to back down and war was only averted by the signing of the Treaty of London, which made Luxembourg neutral and stipulated the demolition of the fortifications.

Over the following decade large parts of the fortifications were demolished and the foundations buried. The superstructure of the Bock Casemates was razed and the casemates in the rock were blasted open to render them useless, since they could not be demolished.

Visiting Luxembourg

Despite the demolition work carried out after 1867 there is still plenty to see of Luxembourg's fortifications. Of the western fortifications, the lower storey of the Lambert Redoubt was uncovered during the construction of an underground car park.

It has been restored and now sits in the midst of the park that covers the area where the western defences once stood. The Louvigny Redoubt has similar remains, although it has been spoilt somewhat by the construction of a tall building on top of it.

The defences lining the Pétrusse valley were supporting the town so they were not demolished. Instead the loopholed walls on the top were removed and the entrances to the casemates walled up. The front of the citadel facing the town was removed, but the rest has survived.

The stretch of walls leading from the citadel to the Bock have likewise survived, but their loopholed walls have been removed. The walk along this section is known as the Chemin de la Corniche and is called the "most beautiful balcony in Europe" because of its views.

The Bock Casemates, being carved out of the rock, were impossible to destroy so the openings were simply blasted open using explosives. Most of the superstructure is gone. Both the Bock and Pétrusse casemates are open to visitors in the summer.

North of the Bock, some of the walls along the top of the cliff have survived, including some ornate Spanish sentry posts'that are mostly reconstructions.

The Vauban Towers and the wall linking them is in a fine state, having been recently restored. The rear part of Fort Niedergrünewald, including the Austrian redoubt, still exists, but the upper parts have gone. The wall linking the two Grünewald forts is gone, although the much-altered Grünewald Gate still stands. Only the upper parts of Fort Obergrünewald were demolished and restoration work was almost complete when I visited (September 2008).

At Fort Thüngen, the towers survived along with the foundations of the rest of the work. The fort has been completely rebuilt and now forms a museum of the fortress. There is a relief map of the town and its fortifications in a unique display under a glass floor.

The medieval walls across the Rham Plateau have survived, along with Vauban's advanced work, the Rumigny Redoubt. The communication walls either side of the plateau (linking it to the Bock and the hornwork) and the rear part of the hornwork also remain.

All in all, given the demolition in the 1870s, a surprisingly large part of Luxembourg's fortifications has survived. Many of the remains have been restored to their original appearance, often rebuilt from their foundations (and probably in contravention of the Treaty of London!) A comprehensive visit of the fortress takes a full day and involves lots of steep climbs, but is well worth it for the views.

Despite the demolitions, Luxembourg remains one of Europe's most impressive fortresses from the era of bastioned fortification. The old town and the fortifications are about 10 minutes' walk from Luxembourg station. There are regular international trains from Paris, Metz, Brussels and Trier.

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