During the conflicts between France and Spain, the French had repeatedly attacked the land near Namur via the so-called Gap of the l'Oise between the fortress of Avesnes and the Meuse. In the 1540s Mary of Hungary, the governess of the Netherlands, decided to build a new fortress to protect this area. The new fortress was built on marshy ground close to the newly-built French fortress of Maubert-Fontaine a short distance to the south. The town, named Mariembourg after Mary of Hungary, was created from an existing village and surrounded by the fortifications.

The town took the form of a large square, with an arrow-headed bastion at each corner and streets radiating out from a central square. This was unusual for early planned towns, which were mostly pentagons (such as Rocroi). There was a single gate, the Porte de France, which faced south-west. The walls were surrounded by a ditch that was flooded from the nearby streams, but there were no outworks.

The bastions were typical of early the Italian School of Fortification, with recessed flanks and squared-off orillons. Each flank had a lowered area in the flank, so that the guns there would be hidden and protected from enemy fire. Maps of Mariembourg in the 16th century show a small building on each of the four bastions. It is unclear what the purpose of this building was, but it could have been a guardhouse or a small powder magazine.

In order to encourage the town to grow, the soldiers in the garrison were allowed to build houses there. A church and a well were also built for the new town. The streets were laid out in order to allow quick access to the ramparts from the centre of the town. This meant that troops could be mustered in central square and quickly dispatched to any part of the walls. However, it created problems for houses built at the angles of each block, so planners of later towns tended to favour a grid plan.

The fortress was finished in 1546 and it was soon put to the test. In 1554 a French army advanced north and headed for Avesnes, causing the Spanish to believe that this was their target. However the French then changed direction and laid siege to Mariembourg. Under strength an unprepared, the garrison surrendered after less than a week. The Spanish reacted by hastily constructing two new fortresses farther north to stop the French advance; Philippeville and Charlemont (Givet).

Under French occupation Mariembourg was renamed Henribourg after the French king. There were several Spanish attempts to retake the fortress, but they all ended in failure. Nevertheless a few years later in 1559 Mariembourg (as it became known once more) was returned to Spanish control by the Treaty of Cateau-Cambrésis. For the next 100 years it remained Spanish and was not attacked - the Gap of the l'Oise was now protected by Mariembourg, Givet and Philippeville, which were an effective deterrent to French forces.

In 1659 the Treaty of the Pyrenees finally transferred Mariembourg to France permanently. The 17th century saw modifications made to the fortifications, including the construction of four demi-lunes between the bastions and a covered way on the outer edge of the ditch. Vauban surveyed the fortress but disapproved of it. It was however maintained and garrisoned as a fortress in his Pré Carré system of defence for the northern frontier.

The fortress saw action for a final time in 1815 after the Battle of Waterloo when the garrison made an impressive but ultimately futile resistance against a Prussian force. In the 1850s the fortifications were demolished and today there is no trace of them apart from the street plan of the town.

The 3D model shown in the images above has been created to represent Mariembourg in the 16th century when the fortress was first built, based on contemporary plans.

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