The Citadel of the Autarch

Book 4 of Wolfe's 'Book of the New Sun' series

What a crazy ride this was. Gene Wolfe started out with a bonkers story and accelerated to the end. It's the same fantasy world all the way through but the SciFi element increases through book 4.

Although the series is split into four novels, and sold as two books, it's one story and these days would probably be edited down a bit and sold as one book.