Situated on the border of three provinces; Flanders, Artois and Hainaut, and an the highest navigable point on the River Scarpe, Douai's strategic importance resulted in its fortification from early times.
The town was fortified almost continually from the 11th century to the 16th century, boasting an impressive set of medieval walls, towers and gatehouses. To adapt the defences to artillery defence, the Spanish erected 19 earthwork demi-lunes between 1627 and 1649.
The town already had 3 bastions on its northern side, but the Spanish opted not to build more bastions but to protect the medieval walls by building outworks in front of them. The fortifications were completed by a flooded ditch and a strong covered way.
From 1641 to 1646 an earthwork pentagonal bastionned fort was built on the north at a distance of 1.5km from Douai, because the inhabitants had opposed the building of a citadel in the town.
This fort, built on the left bank of the river, was known as the Fort de Scarpe. It acted as an advanced work and also held sluices that could divert water from the river to cause inundations on the surrounding ground.
In 1667 Douai surrendered to the French after just 4 day's of siege, due to the fact that the Spanish garrison consisted of only 700 men! Vauban inspected the fortifications shortly afterwards and made some minor alterations to the trace, raising the height of the rampart in places. Later in the 17th century, several more demi-lunes were built.
Vauban recommended that the Fort de Scarpe be demolished, reasoning that the cost of revetting it in stone, constructing demi-lunes and building barracks was not worth the small advantage the fort gave. However, when Louis XIV visited the fort, he followed Prince Condé's advice to keep the fort.
In a memo written around 1669, Vauban described the strength of Douai, saying that the relief of the ground "obliged an enemy to make lines of circumvallation much larger and on more difficult terrain than at Arras [a nearby fortified place]".
The Fort de Scarpe was revetted in stone, three demi-lunes, a covered way and barrack buildings were constructed. Work began in 1670 and the fort was finished in 1672, having cost the treasury over 130,000 livres.
More outworks were built around the town throughout the 17th century. By 1693 there were 24 demi-lunes, although only 5 were revetted in stone. The additions continued well into the 18th century - by 1776, there were around 60 outworks in Douai's defences.
The fortifications were declassified in 1889 and were demolished soon afterwards to allow the town to grow. The Fort de Scarpe was also demolished, although the reason why remains a mystery, as the site was not urbanised.
The demolition of the fortifications left only the Valenciennes and Arras gates intact, as well as a few fragments of the medieval fortifications, including two medieval towers - the Tour des Dames and the imaginatively named Tour 63.