Anantes, Anetans, Nante, Nantoel, Natanc, Nauntes

So [the eleven rebel kings] consented together to keep all the marches of Cornwall, of Wales, and of the North. So first, they put King Idres in the City of Nauntes in Britain, with four thousand men of arms, to watch both the water and the land.

Nauntes must have been strategically important, though this seems to be the only time Malory mentions it.

There is, of course, a city names Nantes in Brittany, and Brittany is sometimes called Britain in the old books; nevertheless, the context of the passage suggests that here a city in Great Britain is meant.

In present-day Merioneth County, Wales, is a point of land called Cader Idris; it is on the coast, immediately south of the inlet at Barmouth. The name Idris suggests the name of the king delegated to hold the city, making Caer Idris a candidate for “the City of Nauntes in Britain”. Chrétien de Troyes, himself French and apparently proud of it, saw nothing strange about Arthur holding court across the Channel of Brittany.

In Middle High German romance, Brittany is often noted as Arthur’s primary kingdom, with its capital at Nantes.

See also
Bishop of Nantes | The Legend of King Arthur