Oostvoorne: place of a Roman fort, destroyed by the sea.
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. The governor of Gallia Belgica, Didius Julianus, defeated them. The Roman government responded by building several forts along the coast of what is now Zuid-Holland, Zeeland and West-Vlaanderen.
Because the sea has been active in the third century, the castle at Oostvoorne, which guarded the estuary of the Meuse ("Helinium"), must be sought somewhere off the coast, in the direction of the Maasvlakte extensions of the Port of Rotterdam, perhaps in the lake that is known as Oostvoornse Meer. The ruins have been seen as late as 1752. A villa complex has been identified at nearby Helhoek.
Because the remains survived for centuries, it is likely that the fort was at some stage replaced by a heavily fortified castle, made of natural stone, and we may assume that this happened in the fourth century.
In the twelfth century, a new castle was built (still visible in Oostvoorne). This proves that the Roman castle was no longer in use and swallowed by the sea.