Complex sentence: The Dukes of Norfolk kept their allegiance to Rome even when Catholicism was outlawed, and their seat at Arundel became a Catholic stronghold: a chapel within Arundel Castle was used for many years, a large church was built in 1870--73.
Split into simpler sentences: The Dukes of Norfolk kept their allegiance to Rome even when Catholicism was outlawed, and their seat at Arundel became a Catholic stronghold. A chapel within Arundel Castle was used for many years until a large church was built in 1870--73.