For ages an island by the entrance of the Amsterdam harbor, named
Pampus, served as a defensive position and as a place where the ocean
sailers anchored. In the late 19th century a fortress was built here,
which, by the time it was finished was outdated already and therefore
never used. For years the fortress was left alone to a slow process of
decay. In the seventies I remember going there with friends to
picknick and party, which wasn't without danger because of the
invisible, overgrown holes and cracks.
About ten years ago a foundation of volunteers started taking care of this forgotten piece of heritage and now there is a regular ferry to the island in the summer with tours given in the fortress.
For more see: www.flickr.com/photos/docman/sets/540340/