Complex sentence: The Earl chose to combine personal concerns with those of the country by attacking the Earl of Winchester's castle at Mountsorrel in Leicestershire--the castle from which the Earl of Winchester's predecessors had ousted Ranulph's grandfather (Ranulf de Gernon).
Split into simpler sentences: The Earl chose to combine personal concerns with that of the country by attacking the Earl of Winchester's castle at Mountsorrel in Leicestershire. It was the castle from which the Earl of Winchester's predecessors had ousted Ranulf's grandfather (Ranulf de Gernon).