Citadel of God is set in Italy of the early 6th century. The Roman Empire is decaying. An Ostrogoth king, Theodoric, is marching in triumph to occupy Italy. A young boy, ward of the respected Roman scholar Boethius, rushes in to stab the barbarian conquerer with a stylus in defense of Old Rome and is rescued from death by a young scholar from Nursia who is called Benedictus.

Against this backdrop, the novel goes on to tell of the lives of Boethius, who becomes governor for the king in spite of his patriotism for the dying Rome; Peter, the young would-be assassin and patriot, who grows up to become a conflicted Byzantine ambassador; and Benedictus, who becomes appalled by the corruption in Roman society and retreats into the wilderness to become a hermit. Eventually he founds several monasteries and travels to Montecassino. He drives out pagan sorcerers, works miracles and founds a monastery that truly became a "Citadel of God" that stood against paganism and Arianism, while the scholar Boethius is jailed and executed by the increasingly paranoid king, after writing The Consolation of Philosophy.

This book is strongly and vividly written, telling an epic tale of several famous lives intertwined during this unstable and critical time in the history of Christendom. The reading and interest level are suitable for ages 14 and up; however, the emotional maturity level may be somewhat higher. There are a couple of scenes, as when a Roman lady tries to seduce the young Benedictus, or a corrupt Christian priest converses with his mistress, that though not overly graphic might be unsuitable to some young readers.