The Grand Risban was a massive stone fort built between the town and the low tide mark. It was the largest of the Dunkirk coastal forts and an impressive feat of engineering. The fort took the form of a triangle with curved faces, reflecting the three specific reasons for its construction; one side faced north to supplement the jetty head forts, one side faced south to cover the sand dunes to the north of the citadel and the third side faced east to cover the foreshore east of the jetties.
The fort consisted of a single battery on the top, mounting 45 guns (15 per side) and there was a garrison of 100 men. In the centre was a sunken courtyard surrounded by barrack buildings and storerooms three storeys high. This arrangement protected the buildings from enemy fire but still allowed for windows facing into the courtyard. The commander's residence was a larger building in the centre of the east side. There were powder magazines in the vaulted cellars beneath the barracks. Three staircases in the corners enabled the garrison to access the battery as quickly as possible. The fort was linked to the west jetty via a wooden bridge that led to a gate in the wall. Through the gate was a tunnel that led directly into the courtyard.
The Grand Risban was an impressive engineering acheivement, but in reality it was poorly situated to cover the three areas that it was built to protect. To the north, the jetty head forts were very effective in protecting the entrance to the channel and the Allies never managed to destroy them, so the Risban was of little value there. To the south, the Risban was too far away from the town, so its guns could not effectively cover the dunes there. The Fort de Revers was built to carry out this role. To the east, the area was too large to be defended by the guns of the Risban from the opposite side of the channel, so the Château Gaillard and later Fort Blanc were built to provide additional protection. It seems that the fort's location was a compromise between these three functions, so that it was unable to carry out any of them effectively. Nevertheless, the Risban was a formidable work that was feared by the Allies. They knew that if they were ever to succeed in destroying the jetty head forts they would meet a much more formidable obstacle in the form of the Grand Risban.