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.

Aire seen from the south.

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-François 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.

View of Aire, and Fort-François can be made out at the back-right.

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-François, 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-François (also called Fort Gassion) is intact, but the bastions were demolished. The internal buildings now house stables.

View from the north-west. Detail of the northern hornwork.
Aire from the north. Detail of the eastern hornwork.
Fort-François, with the town in the background. Fort-François.
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