Thirlestane Castle, Lauder, Berwickshire, Scotland (off A68, just south of the town)
See the website: http://thirlestanecastle.co.uk/

Site of the duel: Kilburn Wells, London, England

Comments made by Lord Lauderdale in the House of Lords were taken by Benedict Arnold as a slight. He demanded a retraction or a duel. When Lauderdale refused to recant, the duel was set for 7AM on Sunday, July 1, 1792 at Kilburn Wells. A full account of the story is found in "Traitorous Hero," by Willard Wallace (pp. 297-299 in the first edition.)