Belvedere Castle is one of the original structures built for Central Park by Frederick Law Olmsted in the late 1800's. It is another gem in Central Park missed by most tourists. Like with much of the Park, most people tend to stay in the southern end of the Park (usually below 72nd Street). But if you do make the trek above 72nd Street, it's worth a visit.
The view is wonderful. Turtle Pond and the greatest view of the Great Lawn. The castle is small but not generally packed with people so access is very easy.
It's really worth the trip to see.