Governor's Island, NY Fort Jay Powder Magazine tunnel down to Magazine Court
Fort Jay (for John Jay the governor of New York) built 1797-98, upgraded in 1803 when it was renamed Fort Columbus (it was renamed Fort Jay in 1904). Major Jonathan Williams designed the new fort with earthworks of granite and brick walls and the footprint of the fort was enlarged.
Williams replaced the earthworks with sandstone and granite walls and a arrow-shaped ravelin all surrounded by a dry moat. The moat is surrounded by a glacis. The glacis was the primary parade field for the fort and also utilized for a polo field. A golf course was placed on the glacis in the 1930s.
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).
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.