Castlereagh refers to Robert Stewart, the Viscount Castlereagh, who was a prominent British statesman and foreign secretary in the early 19th century. He played a key role in shaping the post-Napoleonic order in Europe through his influence in the Congress of Vienna, emphasizing diplomacy and a balance of power. His efforts were foundational in establishing the Concert of Europe, a framework aimed at maintaining stability and conservative values across the continent.