Fort Nassau was built in 1796–97 on Sablica Hill to defend Willemstad and the Schottegat. The fort saw little effective military action, later served as a harbour signal station, and retains much of its original compact form.
It now houses a restaurant, so the interior is not run as a conventional historic site. The hilltop position and wide view over the harbour are the main reasons to visit; reserve if you intend to eat rather than assuming unrestricted sightseeing access.