Caer Dathyl is the main castle, belonging to the Sons of Don, who rule over most of Prydain with a fair and just hand. It is home to the High King Math and to other lords of Don.

Contained within is a small bunch of shops, the Great Hall (where the lords meet with the High King to discuss battles and so forth) and the Hall of Lore (where all of the records of the past are kept).

This castle was destroyed by the Cauldron Born in novel The High King.