Fort d'Ambleteuse

Fort Ambleteuse, also called Fort Vauban (or, incorrectly, Fort Mahon), is a fort located on the coast of the commune of Ambleteuse in Pas-de-Calais in France at the entrance to the estuary of the Slack. It was built by Vauban on the orders of Louis XIV to defend a war port. It is named Ambleteuse tower or Ambleteuse fort in all official documents until the Napoleonic land register of 1803. The name Fort Mahon, certainly due to an administrative transcription error, does not appear until 1840.
Fort Ambleteuse was built at the end of the 17th century, in 1680, at the mouth of the Slack. It is a construction by Vauban.
During World War II, the fort was used to imprison foreign forced laborers.
The fort is sometimes surrounded by the sea at high tide and accessible on foot at low tide. It is the only fort on the coast that has been preserved, thanks to the restorations promoted by Doctor Jacques Méreau and the geologist Destombes who created for this purpose, with a team of volunteers, in 1967 the “Association of Friends of the Fort d 'Ambleteuse' who bought it for the symbolic franc.
The only sea fort still standing from Cherbourg to the Belgian border, it is also a very rare example of a historical monument of this importance, owned by an association which alone assumes the operation and costs.
Since then it has been restored, renovated and opened to the public. The restorations were carried out in three major projects: the rampart supported by the State; the annex buildings restored with the help of the State, the General Council of Pas-de-Calais, Crédit Agricole, the municipality and the owner association; in 2010, a series of projects on all the buildings including the large roof, undertaken with the participation of the State, and aid from Crédit Agricole and the Heritage Foundation.
fr.wikipedia.org/wiki/Fort_d%27Ambleteuse
fortdambleteuse.fr

This fort is the only one left of the seven that defended the coast of Boulogne.
In 169, the English king James II, chased away by his subjects, came ashore in the already silted up harbour. When Napoleon planned an invasion of England, part of his fleet was located here. From the beach you have a view of the entrance to the port of Boulogne and in clear weather even of the English cliffs.