Natrona PA is a very densely built, very small town surrounded on all sides by steel mills, a steep hillside and the Allegheny River. Natrona has a really interesting collection of late 19th century worker housing. The town is mostly intact but many houses are abandoned and seemingly at risk. Natrona is home to the Pennsalt Historic District which is what remains of the company town built for Pennsylvania Salt Manufacturing Company workers in the 1850s. I'm not sure when the steel mills up and down river of the town were built. The mills are currently operated by Allegheny Technologies which is a fairly successful specialty steel maker.
Natrona has very narrow streets and the remnants of a small commercial district which did not seem to have any open businesses when I was there. Natrona has been eclipsed by the much larger and younger town of Natrona Heights on the hill above. It's a very interesting little time capsule and seemed very much at risk.