By far, the most impressive structure is an old blast furnace, which was used up until the 1950s. It towers over the buildings and powerfully dominates the scene.
A photo gives no indication of size. It is, I don't know, 8 or 10 stories high and stretches for a long city block or two.
The details are beautiful.
It's definitely worth a trip. Also in Bethlehem is a well-preserved Moravian district -- they first settled the town and gave it its name.