Complex sentence: A century later this castle was given to William Marshal by Richard I. Marshal, who would become one of the most powerful men in 12th-century Britain, rebuilt Pembroke in stone creating most of the structure that remains today.
Split into simpler sentences: A century later, the castle was given by Richard I to William Marshal, who became one of the most powerful men in 12th-century Britain. He rebuilt Pembroke in stone creating most of the structure that remains today.