The Citadel of the Autarch by Gene Wolfe

The Citadel of the Autarch is the final book in a four-part series set in a far future post-apocalyptic Earth. The protagonist, a man named Severian, is an exiled executioner who has been travelling through a world where society has regressed to a technology level similar to the Middle Ages. Severian becomes embroiled in a war against alien invaders, and his journey ultimately leads him to the titular Citadel where he must confront the Autarch, the ruler of the Commonwealth. The book combines elements of science fiction and fantasy, exploring themes of memory, truth, and the nature of power. The 9899th greatest book of all time