Most stories about knights and castles take place in the distant past. This one is set in the distant future.
Humanity is nearly wiped out by a large asteroid that strikes the moon, and breaks it into many pieces, sending the pieces toward Earth. If the initial earthquakes and tidal waves don't kill people by the millions, the later nuclear winter does.
Thousands of years later, eastern North America (where this is set) has reverted to a medieval level. Harris is the son of Aidyn, King of Pennland. Aidyn doesn't know (or seem to care) about conditions in his kingdom, and won't let Harris get on a horse to find out. Aidyn spends all day in his office, doing whatever monarchs do, and Lucida, his mother, rarely comes out of her rooms. Harris is very bored.
As a teenager, Harris is sent to the neighboring kingdom of Carolinas, to learn how to be a ruler. After several years of fighting battles, and recovering from a severe sword injury, Harris gathers an army and is determined to reclaim his kingdom.
Town by town, and district by district, Harris finds utter devastation. Most places are nearly deserted, because everyone has been enslaved, or killed, by a barbarian/rebel leader named Kain. Harris and the army spend time in each place, helping to rebuild. He shows the people that he is for real, instead of merely ordering them to obey them. Even then, some people are very reluctant to accept him as king. When he gets home, he finds more utter devastation. Aidyn is dead, poisoned by Lucida, who has descended into full-blown insanity. The castle itself is beyond filthy and disgusting. It is as if a combination of an invading barbarian horde, and the ultimate in wild and drunken parties, happened several months ago, and no one has bothered to clean up. Will Harris be able to remove Kain, the cause of all this, once and for all?