Casus Belli Sources
The following GURPS sourcebooks are available upon the beginning of the campaign:
Basic - Derp.
Camelot - mostly for flavor and references. Players are advised to take everything in that book with a serious grain of salt.
Celtic Myth - mostly for flavor and an idea of what the culture is like. The GURPS: Casus Belli campaign begins centuries after the mythological era referenced in this book.
Imperial Rome - as with Celtic Myth, this is mostly for flavor given 5th Century Prydain has mostly moved back toward their tribal tendencies. Major cities such as Londinium will utilize this more than anything else.
Low Tech - considering 5th Century Prydain is TL2, this is to be expected.
Martial Arts - The following martial arts styles are available: Armatura, Armatura Equestris, Dagger Fighting, Hoplomachia, Quarterstaff, Heroic Spear Fighting, Viking Spear Fighting, Sword-and-Shield Fighting, Combat Wrestling, No Asian styles.
Note that other supplements will be integrated as appropriate.