The town of Aire-sur-la-Lys was fortified in medieval times with a strong castle and walls around the town, fronted by a flooded ditch. Some crude bastions were added to the defences in the 16th century, followed by the construction of some demi-lunes.
The 17th century saw more improvements to the fortifications, so that Aire had an impressive trace of 8 bastions and 10 demi-lunes and 8 hornworks, but it still fell to the French in 1676. In 1642 the Spanish built a small pentagonal fort called Fort Saint-Franšois 2km east of the town. This fort protected the eastern approach to the town and guarded a canal junction. Vauban demolished most of the hornworks, deeming them superfluous.
He improved the trace in some places but put the emphasis of the town's defence on inundations on the low-lying ground surrounding Aire. At Fort-Franšois, Vauban made a minor alteration to the trace and added a covered way.
The fortifications of Aire were mostly demolished in the 19th century, but the Bastion de Thiennes (the south-west bastion) remains as does the water gate in the north-west. The gate of Fort Saint-Franšois (also called Fort Gassion) is intact, but the bastions were demolished. The internal buildings now house stables.