It is mentioned in the Welsh tale Culhwch and Olwen whose manuscript dates from the 11th century, though the story is much older.
The story describes the court as being at Celliwig in Cernyw (the Welsh name for Cornwall), otherwise known as the kingdom of Dumnonia including modern Devon.
The hall is guarded by Glewlwyd Gafaelfawr, Arthur's porter, and Culhwch has difficulty gaining entrance due to the special laws that restrict entry once a feast has begun.
It describes Arthur's warriors at the court in depth and says that: "From here, one of his Warband, Drem, could see a gnat as far away as Scotland; while another, Medyr, could shoot an arrow through the legs of a wren in Ireland!"
[1] In the Iolo Manuscripts (1843), a corpus of pseudo-medieval Welsh texts by the renowned literary forger and inventor of tradition Iolo Morganwg (1747–1826), Celliwig is referred as the former site of the "throne of Cornwall" but the text adds that it is now at Caervynyddawg (Caerfynyddog), a site which is otherwise unattested.