This is Bishop's castle. It was built entirely by one man

Strongly reminds me of the Palais idéal du facteur cheval in Hauterives, France !

A "palace" that was singlehandedly built by a postman over thirty years (late 1800s) with pebbles mainly picked up on his route.

https://www.facteurcheval.com/wp-content/uploads/2021/02/Guide-de-visite-UK.pdf