If you take the A train you can get to 190th from Penn Station in about 20-25 minutes. You can walk up here from downtown/Penn station area in probably an hour or so, or take a bus. The area around the Cloisters is heavily wooded and perfect for a guy/girl who knows how to burrow into the undergrowth and clear out space for a tent. The area seems pretty isolated but the subway lets out directly at the park and civilization is only a short walk away if you need cigs or food. If you have a camo bivy I imagine you could stay there for awhile, undetected. I thought about this area as a place to live in Manhattan while looking for work downtown, and a room somewhere else in the city that is actually affordable. Worth a shot and MUCH safer than almost anyplace else in NYC.
http://www.yelp.com/biz/fort-tryon-park-new-york-2
http://en.wikipedia.org/wiki/Fort_Tryon_Park
http://www.yelp.com/biz/fort-tryon-park-new-york-2
http://en.wikipedia.org/wiki/Fort_Tryon_Park
