This splendid old church may look a bit prouder than the ordinary Catholic parish church, and it has every right to its pride: for a little more than a decade, it was the cathedral for the Diocese of Allegheny. In 1876 the rapidly growing Diocese of Pittsburgh was split, with Allegheny (then an independent city) as the seat of the new diocese. It was a bad plan from the beginning: Allegheny had all the wealthiest parishes, but Pittsburgh was generously allowed to keep all the debt. The shockingly un-Christian infighting that resulted ended only in 1889, when the Diocese of Allegheny was suppressed. But a Catholic diocese isn’t that easy to get rid of, and there is still a titular Bishop of Allegheny. He lives in Newark, where in his day job he is auxiliary bishop of the diocese there.
St. Peter’s is just across Arch Street from the National Aviary, a short walk from the North Side subway station.
Addendum: This church was built in 1872; the architect was Andrew Peebles, who also designed First English Lutheran downtown.