Construction work started the same year based on plans drawn up by Quartermaster General Willem Coucheron. Most of the work on Fredrikstad Fortress was carried out by soldiers and conscripted farmers who did their military service at the fortress. Fredrikstad Fortress – the Old Town became the army's most important supply base and mobilisation area in Southern Norway.
Fredrikstad Fortress is an earthworks fortification built according to the old Dutch principle. Broad water-filled moats and large earth ramparts with bastions completely surround the Old Town of Fredrikstad. The fortress is a complete urban fortress with all its fortifications and military buildings intact.
On the landward side, the fortress has three full bastions and two demi-bastions, with projecting emplacements for cannons. In the moats lie the ravelins and outside the main fortress there are outworks such as Kongsten Fort. On the river side, the fortifications consist of a solid stone wall reinforced with earthworks.
Entry to the fortress is gained over the drawbridge and through the main gate or through one of the gates opening onto the quays beside the River Glomma. Very many of the buildings have been used for the same purpose for several centuries. Many big fires in the town have repeatedly destroyed the civil buildings, which have largely consisted of wooden houses. The military buildings are therefore the oldest buildings in the town. They stand together with the civil buildings, and are thereby an integral, but marked part of the overall townscape.
The fortress was closed down in 1903, but the Old Town continued as a garrison town until 2002, when the last soldiers marched out. Since the closure, all the military buildings have been taken over for new purposes, and the old fortress town is still very much a living urban community.