Esquelbecq castle has a square form flanked by eight turrets. A white stone border embedded in the walls at mid height indicating the high power of the lords of the place.
The building, accessed by two bridges, is surrounded by a moat. 
It is assumed that a first wooden castle existed in the 9th century to protect from northern invasions, but there is no evidence that it was at the present location. The last general restoration of the castle dates from 1606, date that could be read on the donjon before it collapse. This restoration allowed to add many windows in the walls whose thickness sometimes exceeds 1 meter, in some places of their base. The castle and its outbuildings were registered on the French supplementary of Historical Monument List, July 24, 1944. 
In September 1984, the collapse of the donjon led to the creation of an Esquelbecq Heritage Preservation Committee which allowed its designation as a Historical Monument. 
http://www.ot-hautsdeflandre.fr/en/decouvrir-les-hauts-de-flandre/historical-heritage/historical-monuments

Hardelot Castle

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Hesdin-l’Abbé is a commune in the Pas-de-Calais department in the Hauts-de-France region of France.
Geography
A forestry and farming village situated some 5 miles (8.0 km) south of Boulogne, at the junction of the D901 (formerly the N1 Paris-Calais highway) and theD240 road. The A16 autoroute forms part of the western border of the commune and the river Liane the southern.
Places of interest
The church of St.Leger, dating from the seventeenth century.
The eighteenth-century chateau d'Hesdin-l'Abbé, nowadays the Hotel Cléry.
A seventeenth-century manorhouse.

Château d'Hesdin-l'Abbé

The Château de Boulogne-sur-Mer is a castle in the French seaport of Boulogne-sur-Mer, in the Pas-de-Calais département. It houses the Boulogne museum.
The castle was built in the 13th century by Philippe Hurepel (1180-1234), count of Boulogne and son of Philip II of France. Following the death of his half-brother, king Louis VIII after a short three-year reign, Hurepel was one of the leaders of a rebellion against the regent, Blanche de Castille, mother of the minor Louis IX. He constructed castles at Calais and Hardelot and refortified Boulogne. The castle is built in the eastern corner of the medieval walls surrounding the Haute Ville (literally, high town - the part of Bologne on the hill). The walls themselves were reconstructed by Hurepel. The eastern part of the castle was built over a corner of the Roman wall, parts of which are still visible in the basement. Housing together the political, legal and economic powers of the time, it was also a residential and defensive site.
Various modifications have taken place. Major alterations were carried out by the duc de Berry between 1394 and 1416. The horse shoe shape (barracks, arsenal) was completed around 1567. After being adapted because of developments in artillery during the 16th century, it lost some of its medieval character. In 1767, it became a barracks and, after World War II, it also housed a prison. In 1974, the town council took over ownership of the castle and decided to install its museum collections.
Unlike many other contemporary castles of the early 13th century, the plan of the castle does not include a keep. The remains of Château de Hardelot in nearby Condette show that Hurepel used this design for at least one other castle. A similar castle was built around the same time at Fère-en-Tardenois by the Counts of Dreux. The Château de Boulogne is listed by the French Ministry of Culture as a monument historique.

Château-musée de Boulogne-sur-Mer

Fort Mahon is a sea fort by the commune of Ambleteuse in the Pas-de-Calais, northern France. The fort was built at the end of the 17th century by the military architect Vauban on the orders of Louis XIV to defend the port in the estuary of the Slack. It was designated a Monument historique in the 1960s.
Name
The original title of the fort or tower is Ambleteuse in all official documents including the Napoleonic cadastre of 1803. However the name Fort Mahon, which is used in English, did not appear until in 1840 and is probably the result of an administrative or transcription error.
On October 19, 1965, the government of France classified the fort as a historical monument. However the decree stated that although the fortification was named Ambleteuse it was also sometimes called Fort Mahon. Demonstrating that the misnomer remained in circulation. The private heritage group that manages the site adheres to what they consider to be the official name: Fort of Ambleteuse.
History
Work to build Fort Ambleteuse at the mouth of the Slack was completed in 1680. Vauban sited the fortification at a point at which access can only be made at low tide. Its defences consisted of a five-piece coastal artillery battery situated within a stone-lined bastion (or casemate) with seaward facing embrasures set in walls up to 3 m (9.8 ft) thick. An outer sea wall provided further protection for the inner scarp's gun terrace. In 1945, two sea mines exploded destroying the outer defences. But the wall has been restored in its original form with battlements.
To the landward side, Vauban built a small parade ground and living quarters for the fort's garrison. The gun platform within the bastion was once was a large open space but this was cut in half by a concrete partition that was added by German forces during the Second World War. A pillbox was also added to the inner side of the fort to strengthen its landward defences. The landward walls had embrasures for muskets and small cannons.
Preservation
Ambleteuse is now the only preserved coastal fort still standing between Cherbourg and the Belgian border. It was saved by a geologist named Dr Jacques Mereau Destombes. After recognising its historical imporantance, he recruited a team of volunteers in 1967. The Association des amis du fort d'Ambleteuse (Association of Friends of the Fort Ambleteuse) then bought the fort for a symbolic franc from the French government in the early 1970s.
Restoration has been carried out in three major projects. Firstly the ramparts were rebuilt with state assistance. Ancillary buildings were then restored with the help from the state, General Council of Pas-de-Calais, Crédit Agricole, and the Fondation du patrimoine (France's equivalence to the UK's National Trust) In 2010, all the buildings were re-roofed large roof with funding from the government and several heritage organisations.
There is a street in Bewdley, Worcestershire, named after the Fort : "Fort Mahon Place".
Visiting
The fort is open to visitors every Sunday from April to November. There are two exhibition rooms with orientation plaques throughout the buildings. The viewing platform at the top of the tower has an interpretation board that explains the view across the Straits of Dover.