The fort island of Hinderdam is right in the middle of the river Vecht, so both banks were easy to defend. A defence work has stood on this site since the 17th century. That defence work did not consist of much more than an earthen sconce. Nonetheless, it was incorporated into the Dutch Waterline soon after it was built. Its isolated position and the fact that it has not been in use since the middle of the last century have allowed the fort to become host to exceptional plant and animal life. In 1990, the entire island was declared an official natural reserve.