Complex sentence: It mostly developed in the late 19th and early 20th centuries, and it features a number of large houses that were built for the city's steel magnates, as well as, most notably, a large church, St John's.
Split into simpler sentences: It mostly developed in the late 19th and early 20th centuries. It includes a number of large houses that were built for the city's steel magnates, and most notably, a large church, St John's.