In medieval times the town of Montreuil-sur-Mer stood on a hill near the mouth of the river Canche. An important port, it was first fortified in the 9th century, when a stone wall was built around the town. In the 13th century the town was expanding and so new walls were built to enclose it. The new circuit of defences extended farther south to take in a new market square and farther east and north down the hill to take in the lower town, which was growing up on the riverbank. At the north-east corner of the town a castle with 8 towers was built, forming part of the walls. However, Montreuil's fortunes declined as the estuary silted up and much of the lower town was not built up.
In the 16th century the town was on the northern frontier of France, with the Imperial province of Artois just to the north. Montreuil saw several sieges in the wars between the French and the Habsburgs and their English allies. The first came in 1522, when a joint Imperial and English force failed to capture the town. The second, in 1537, was more disastrous. This time the besiegers captured the town and burnt it, destroying many of the houses. As a result, the French decided to strengthen the fortifications. The work was carried out by an engineer called Jean Marin. Marin saw that the weakest part of the town was the north and east sides where the walls were at the bottom of the hill rather than at the top. He built a new rampart in the north and east, inside the old walls. The result was an inner circuit enclosing the upper town.
The new wall had 5 bastions, which were among the first to be built in France. Below them, the 13-century walls around the lower town were left in place to serve as an outer line of defence. When the English laid siege to Montreuil in 1544, the work on the new fortifications was sufficiently advanced to deter them from attacking from the north or the east. Instead, they attacked from the south, where the approach is less steep, but they were unsuccessful. After holding out for 3 months, Montreuil was relieved by a large French army that forced the English to retreat to Boulogne. Work on the fortifications continued and was completed in 1549.
In 1567, under French king Charles IX, the fortifications were strengthened by the construction of a bastioned citadel built around the castle. Following to Spanish attack in 1596, King Henri IV ordered his military engineer Jean Errard'to strengthen the fortifications of Montreuil. Finally, the 13th-century wall around the lower town was reinforced by the construction of 3 demi-lunes. Thus, in the early 17th century Montreuil was protected on all sides by artillery fortifications.
Monrtreuil was again strengthened in the 1630s when Chevalier de Ville, Louis XIII's military engineer, had the walls backed with earth to enable them to absorb cannon-fire more effectively. The old round towers were lowered and adapted for artillery and muskets.
Vauban'visited Montreuil twice in the 1670s to survey the fotifications. He disapproved of the citadel and even suggested demolishing it and building a bastioned front in its place. In the end he added a demi-lune to the town side instead. Vauban was also responsible for three earthwork demi-lunes on the east side of the upper town and for the covered way'surrounding the defences. Finally, he put sluice gates in place for inundations'and built three bastions and a hornwork to the north of the lower town.
In 1845 the north bastion of the citadel was rebuilt with Haxo casemates and in 1867. The fortifications of the town were declassified, but the citadel was garrisoned until 1929. For the most part the fortifications are in good condition, although the earthwork demi-lunes on the east side of the upper town are rather overgrown. The citadel, which is currently being restored is normally open in the summer months, with a small entrance fee.References:
First record of Kastelholma (or Kastelholm) castle is from the year 1388 in the contract of Queen Margaret I of Denmark, where a large portion of the inheritance of Bo Jonsson Grip was given to the queen. The heyday of the castle was in the 15th and 16th centuries when it was administrated by Danish and Swedish kings and stewards of the realms. Kastelhoma was expanded and enhanced several times.
In the end of 16th century castle was owned by the previous queen Catherine Jagellon (Stenbock), an enemy of the King of Sweden Eric XIV. King Eric conquered Kastelholma in 1599 and all defending officers were taken to Turku and executed. The castle was damaged under the siege and it took 30 years to renovate it.
In 1634 Åland was joined with the County of Åbo and Björneborg and Kastelholma lost its administrative status.