Viscount Castlereagh was a courtesy title only, since he was actually the heir to the Earl and (later) Marquess of Londonderry. Our marquess succeeded to the titles in 1821. (He died in 1822, without children, and was succeeded by his half-brother.)
The family seat is Mount Stewart, in Northern Ireland, close to Strangford Lough. This is the imposing front entrance which faces the park and the path to the huge lake.
Sadly, the splendid garden is modern, created in the 20th century, and much of the house celebrates the lives of the 20th century family. But you can see how magnificent the interior was. Here, for example, is the marble hall with the galleried landing above.
Some copies of Castlereagh memorabilia are still on view, including paintings, etchings and papers, as well as busts and miniatures. Perhaps most interesting are these chairs. There are 22 of them and they were used at the Congress of Vienna. They were later presented to Castlereagh's half-brother (the 3rd Marquess of Londonderry) as a tribute to the diplomatic achievements of both brothers.
I was disappointed to discover that the needlework is not original! It was done around the end of the 19th century by nuns in Nantes but it does depict the coats of arms of the delegates at the Congress and the countries they represented. And it still sent a shiver down my spine to look at them.