The photo above, from around 1900-1905, is titled “Union Square from Tiffany’s.” The Tiffany store moved to 15 Union Square West in 1870, so we’re looking east from the corner of 15th Street and Union Square West. The overall view is exactly what you’d expect for that era: the trees of the small park, the surrounding road, some horse manure.
The tall building on the far left is the Jackson Iron Works office building on Union Square North, AKA 17th Street. Despite the fact that the Jackson company provided modern steel framing for big buildings, their headquarters was a somewhat backward-looking hybrid structure, with masonry shear walls and cast-iron columns. In other words, that building made it into The Structure of Skyscrapers based on its height, but it is not of any great structural interest.
The thing that jumps out at New Yorkers who know Union Square, but is invisible to everyone else, is that the park is flat, level with the adjoining streets. Union Square, today, is several feet above street grade, so that entering it from any side involves going up a ramp or stairs. What happened?
Right about the time this photo was taken, the original IRT subway was constructed, passing by Union Square. As much as was possible, the IRT was built under public streets, using cut-and-cover construction, so its route ran up Fourth Avenue from the south, along Union Square East, and then the continuation of Fourth Avenue (now Park Avenue South) north of the square. In other words, the IRT subway abuts the park but does not go below it. About ten years after the IRT, the BMT subway, the Broadway Line, was constructed. It runs in a straight line from Broadway south of the park to Broadway north of the park, which means it runs diagonally across the center of the park. I’m not sure when the passageways connecting the BMT and IRT stations were built – I’ve seen dates from the 1910s to the 1930s, but they are at the mezzanine level above the tracks and platforms.
In short, the BMT subway was constructed too high up. In order to be able to build the mezzanine, including the connecting passageway, it was necessary to raise the park. The park was actually raised several feet higher than it is now, basically becoming a planted lid on the station below. During a 1980s renovation that doubled the width of the connecting passageway and created the entrance pavilions to the subway station, the park was lowered to its current level. There’s no way to lower it back to street grade without reconstructing the entire BMT station and the passageway, as well as some of the tunnel north and south of the station, in order to lower the platforms to allow for a lower mezzanine above. In other words, it’s never going to happen.