Complex sentence: Mary married Henry, then known as Bolingbroke and at the time not in direct line of succession to the throne, on 27 July 1380 at Arundel Castle when she was perhaps little more than twelve years old.
Split into simpler sentences: Mary married Henry--then known as Bolingbroke and at the time not in direct line of succession to the throne--on 27 July 1380, at Arundel Castle. At the time of her marriage, Mary was perhaps little more than twelve years old.