Horace Walpole has been called the father of gothic and I believe The Castle of Otranto (1764) was the first Gothic published by an English author (?)
I am trying to make sense of how Otranto originated with Horace Walpole, MP in the first place.

The First edition claimed Otranto to be a translation of an original medieval work.
Attributed 'Onuphrio Muralto, Canon of the Church of St Nicholas at Otranto', 1529. Italy.

Possibly, at the time Walpole wrote Otranto in the 1760s' he was taking advantage of a cultural interest in the medieval past and the Continent.
Wealthy people made the Grand Tour of the Continent, including Italy.
Jane Austen's brother Edward did the tour.

On their return home, English often built neo-Gothic structures inspired by what they had seen on the Continent like follies (including Blaize Castle mentioned in NA as an 'ancient castle' by John Thorpe but really a small C18th building !), faux ruins and Gothic influenced the architecture of some mansions.

Perhaps Walpole thought representing the book as a medieval relic more amusing or trendy with the English public.
And of course, he wanted to see if the public accepted it.

Yet I wonder if Walpole, MP was serious when he wrote TCoO.
The style of Otranto's language may have been a burlesque on medieval literture.
The stilted, melodramatic language is quite funny and appears to rely on momentary surprises rather than terror. I'm finding it abit hard to judge as a C21st reader.

Maybe many C18th readers took Otranto too seriously! ;)