The round building, to the left in the first sketch, is, I believe, Castle William, a fortification built in the 1800s, on Governors' Island, off the southern tip of Manhattan, and near the Statue of Liberty. It is still there.
http://www.wirednewyork.com/guide/go...n_9march03.jpg
http://www.dmna.state.ny.us/forts/fo...liamCastle.jpg