Complex sentence: As part of the foundation of the University, behind which he was the driving force, he moved his residence from Durham Castle to Lumley Castle, and gifted the castle to the University, where it became the home of University College.
Split into simpler sentences: As part of the University's foundation, behind which he was the driving force, he moved his residence from Durham Castle to Lumley Castle. He gifted the castle to the University, where it became the home of University College.