The pastel city (Doubleday science fiction)

Via Wikipedia: The first book The Pastel City, presents a civilization in decline where medieval social patterns clash with the advanced technology and superscience energy weapons that the citizens of the city know how to use but have forgotten how to engineer. Harrison's leading character, Lord tegeus-Cromis, fancies himself a better poet than swordsman; yet he leads the battle to save Viriconium, the Pastel City, from the brain-stealing automatons known as the geteit chemosit from Earth's past.