Question: Which ruined castle in South Lanarkshire, Scotland overlooks the River Nethan?
Sentence (from "River Nethan"): The River Nethan is a river which flows between Glenbuck in East Ayrshire and Crossford, South Lanarkshire where it feeds into the River Clyde, with the town of Lesmahagow and surrounding villages sitting on the river course.
Relevant evidence: No