The Château d'Espérance was a large wooden fort built at the head of the west jetty. It took the form of a wide curved battery facing out to sea. Behind the battery was a long building with accommodation for the garrison, stores and a powder magazine. There were two demi-bastions to protect the rear of the fort, although no cannon faced the rear. The whole fort stood on a forest of stilts, which were driven into a substantial piling base to secure the foundation in the shifting sand beneath. There was a line of vertical stakes on the seaward side to stop fireships from running into the fort. A wooden bridge linked the fort to the jetty itself.
The Château d'Espérance covered the deep water between the low tide mark and a sandbank that lay off the coast, called the Brack. The Brack was just within range, so a ship could not sail past the end of the jetties without coming under the fort's guns.
During the Anglo-Dutch attempts to bombard Dunkirk in the 1690s, the Château d'Espérance bore the brunt of the attacks. In 1694 the Allies sounded the approaches and realised that it would not be possible to bombard Dunkirk without destroying the forts at the jetty heads first. Fireships were sent against it, but they were all sent off course by the waves or by French longboats towing them aground. During the 1695 attack Jean Bart, the famous Dunkirk privateer, was put in command of this fort.