Complex sentence: In 1964, Chepstow Rural District Council bought the castle from the Cobb family for £12,000, at a time when the increasing availability of council housing had supplanted the local need for the castle apartments.
Split into simpler sentences: In 1964, Chepstow Rural District Council bought the castle from the Cobb family for £12,000. The increasing availability of council housing at that time supplanted need for the castle apartments.