The Fortress can be visited
FortressWillemstad
Willemstad Fortress

In 1561, Jan IV van Lymes, Marquis of Bergen op Zoom, commissioned the drainage of the salt marshes in the extreme north-west of his marquisate. The polder was completed in 1564 and was named the Ruygenhil.
Source: vestingsteden.nl
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Location: 51.6920, 4.4380