The Pastel City (Viriconium #1)

The first book in the Viriconium series: In the distant future, a medieval system rises from the ruins of a technology that destroyed itself. Armored knights ride their horses across dunes of rust, battling for the honor of the Queen. But the knights find more to menace them than mere swords and lances. A brave quest leads them face to face with the awesome power of a complex, lethal technology that has been erased from the face of the Earth--but lives on, underground.