My copy (Barnes & Noble edition, 2004) has some interesting background information about Horace Walpole, written by Annie Pécastaings (Ph.D., English, Tufts University), which I hope is all right to share:

Apparently in June 1764, Walpole had a vivid, haunting dream. The next day, all he could remember was that "I had thought myself in an ancient castle ... and that on the uppermost banister of a great staircase I saw a gigantic hand in armour." He then "sat down, and began to write, without knowing in the least what I intended to say or relate."

Dr. Pécastaings also suggests that Otranto may have been partially inspired by Walpole’s experiences as a politician. In April 1764, Walpole’s cousin and best friend, Henry Conway, lost the command of his regiment and his appointment at court. Horace Walpole was so upset that he withdrew to his country house for a time, and in June he had his dream and wrote his novel.

Walpole already had an interest in the Gothic; his country house, Strawberry Hill, is a "little Gothic castle" with all kinds of towers, battlements, etc. and filled with a variety of art and antiques. While looking through the web sites below, I couldn’t find any evidence of secret passages (although perhaps if we knew about them, they wouldn’t be secret, right? :) ). Has anyone participating in the GR toured Strawberry Hill? How similar is it in layout or other details to Otranto?

I Googled around and found a history of the house itself:
http://www.twickenham-museum.org.uk/detail.asp?ContentID=174

A more general travel guide can be found at:
http://www.guidetorichmond.co.uk/strawberry.html

Best of all is the web site for the "Friends of Strawberry Hill":
http://www.friendsofstrawberryhill.org/
which has a nice picture of the house at night, interior photos, a history of the house – this site is a real treasure trove. I'm delighted to see they host a scary movie night on Halloween. :)

As a final thought, Dr. Pécastaings cites a letter from Walpole to Madame du Deffand, the famous French salon leader: