Caer Cadarn is ruled by the "rotund"
King Smoit. This castle rules much of the area around it, and includes the areas ruled by Lord Gast and Lord Goryon (who fight over cows and land all the time).
This area is first encountered in the novel The Book of Three. In The High King, it is the location of Magg's first attack, in which he takes over the castle. He is over thrown by the machinations of King Smoit, Taran and Company.