No, I'm not trying to say that Calvert Vaux, the co-designer of Central Park, was foolish; perish the thought!
In architecture, a folly is a building designed and constructed purely for the delight of those who behold it. Belvedere Castle is exactly that, and has delighted visitors to the Park since 1869.
Strictly speaking, an architectural folly is built for no practical purpose, but since 1919, the National Weather Service has operated a weather station at Belvedere Castle. You can just about make out some of the instruments, above the peak of the main turret, in this photo. So next time you're in New York City and here a radio weatherperson says, "... and the temperature in Central Park is...", you'll know where it comes from.
Belvedere Castle is a major stop on my Central Park photo safaris, as the view from its ramparts is stunning. Also, the Castle itself, perched high on Vista Rock, is photo-worthy both from below and close-up.