It is. Built by a postman called Mr Cheval (means "horse") who picked up stones each day while delivering mail by foot, then went crazy about building this. The guy was persevering for sure. Truly unique "palace" indeed, you won't see that anywhere else.

Really interesting story.

Just before his death, Cheval received recognition from figures including André Breton, Robert Doisneau, and Pablo Picasso.[10] His work is commemorated in an essay by Anaïs Nin. In 1932, the German artist Max Ernst created a collage titled The Postman Cheval. ... In 1969, André Malraux, the minister of Culture, declared the Palais a cultural landmark and had it officially protected.