A city probably in Wales.
Welsh oral tradition had it that Arthur was imprisoned in Caer Oeth for three days by Gwen Pendragon before he was transferred to another prison.
Notes
Caer is a Welsh name for a wall or mound for defence – a city or castle wall, a fortress.
The root to this word might be cau, to shut up, to close, to fence, to enclose with a hedge. Cue means a field enclosed with hedges. When the Britons began to build cities they built a fortified wall to surround them, which were called caer.
The name Chester is a Saxonized form of the Latin castruni, a fort (and one of the few words recognised as directly inherited from the Roman invaders), is a common prefix and suffix in English place-names, such as: Colchester, Manchester, Chesterford, Chesterton. In the Danish and Anglian districts “Chester” is replaced with “caster”, such as: Doncaster and Lancaster, but both forms are allied to casirum, a Latinization of the Celtic caer.
Source
Triads of the Island of Britain (Welsh ”Triads”) | 11th century to 14th century