<p>Jonathan Williams began construction of Castle Williams in 1807 and was completed in 1811 - It's a three level 100 gun casemate. It was the first casemated fort in North America. It was part of a larger system of forts such as Fort Wood, Fort Hamilton,and Castle Clinton protecting New York Harbor and New York City. [Aerial of Castle Williams from 1912 located at the Library of Congress.]
</p><p>The northern half of Governor's Island is a National Register historic district #85002435. It's also a National Historic Landmark. The National Park Service runs Castle Williams and Fort Jay as a National Monument. The rest of Governor's Island is administered by the Governors Island Preservation and Education Corporation. The island is required by law to have no housing (which is a shame - I think the Presidio model works very well).
</p>
The island is open to visitors only on the weekends from spring through fall. If you're ever in New York City, I would highly recommend taking the free ferry over to the island and explore. You can even rent a bike on the island.
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/45923298@N00">Adam</a> from Champaign, Illinois, USA
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/army_arch/3308995257/">Governor's Island, NY Castle Williams from the air 1912</a>