Ansonia Hotel
I used to live a few blocks away from the 74th and Broadway Ansonia (then a low price hotel, now an expensive condos.) The film depicted the pre-gentrified place perfectly, and I'm sure was shot on location.
The building was designed for musicians, built in 1904, and had super solid walls and floors to kill the sound. It was originally expensive apartments, and then when the neighborhood declined, they were cut up into smaller units, something alluded to in the film.
And Willy's comment that he could look out his window and see muggings, was not far from the truth. Look up the film "The Panic in Needle Park." The park is a triangle, about a half acre that is right across the street from Willy's window.
The film was delightful, and spot on in every way.