Aardenburg
The location of Aardenburg (number 55)
In 173, the Chauci, a tribe that lived in what is now called Groningen and Ostfriesland, and was well-known for its sea-faring qualities, attacked what is now called Flanders. They had some succes, but in the end, they were defeated by the governor of Gallia Belgica, Didius Julianus. The Roman government responded by building several forts along the coast of what is now Zuid-Holland, Zeeland and West-Vlaanderen. Aardenburg was one of them; it controled the route to Maldegem and beyond. In the third century, the civil settlement grew and a temple was added.
After 274, the fort at Aardenburg was evacuated, probably after an attack by Saxonian pirates, who appeared on the Flemish coast after the collapse of the Gallic Empire, which had been reunited with the Roman empire by Aurelian. Other forts may have had a similar history.
However, in the fourth century, they were in use again, although they had changed into heavily fortified castles. At Aardenburg, archaeologists have excavated the remains of the enormeous walls of 150 x 200 meters (two gates are indicated in the Burchtstraat). The natural stone, which was imported from Tournai and the Eiffel Mountains, shows that the Roman empire was still strong in the fourth century. The castle, which accomodated about 250 soldiers, was probably occupied until the collapse of Roman power in the Rhineland in the first decade of the fifth century.
A satellite photo showing the remains of the excavetd gate can be seen here.