Complex sentence: The castle had fallen into ruin by the 18th century, when the estate passed to the Graham family, then in the 19th century to the Montgomerys, who still occupy Kinross House.
Split into simpler sentences: The castle had fallen into ruin by the 18th century. The estate passed to the Graham family in the 18th century and then, in the 19th century, to the Montgomerys, who still occupy Kinross House.