Most of the books are set in the fictional Kingdom of Gwynedd (pronounced "GWI-neth"), one of the Eleven Kingdoms. The other kingdoms are Bremagne, The Connait, Howicce, Llannedd, Kheldour, Meara, Mooryn, Orsal, Tralia, and Torenth.
The Kingdom of Gwynedd is situated at the heart of the Eleven Kingdoms, being surrounded by the Kheldish Riding in the North, Torenth in the East, the Southern Sea in the South, and Meara, The Connait, and the United Kingdoms of Howicce and Llannedd in the West and Southwest.
Gwynedd and Torenth have a history of conflict that spans centuries.
The church in Gwynedd, and many of the surrounding kingdoms, is very closely based on medieval Catholicism (minus a Pope equivalent), with liturgical services in Latin and using the real world's Bible. Torenth is depicted with an Eastern Orthodox style Christianity, complete with services in Greek, churches with icons, and metropolitans (as opposed to bishops) headed by a patriarch.
The Deryni in Gwynedd have been persecuted by the church for centuries.