Historically a Private Park, only the buildings around it had keys for the gates. This has probably been challenged in court and it looks to me like nothing has changed though.
On one side of the Park (East) are some townhouses, one of which is where Stanford White used to live and have a private studio. (Think it is one most obscured by trees. This area is so leafy and green, it's sort of a shock to come across it.
Inside the park are of course beautiful plantings and neat sculptures.