Caernarfon Castle in Wales is historically significant and also indirectly connected to Poul and Karen Anderson's King of Ys Tetralogy.
In Welsh mythology, Macsen Wledig ruled the Roman Empire from Caernarfon and was the ancestor of the Princes of Wales.
Edward I of England (1272-1307) built the Castle to rival the walls of Constantinople. In 1301, he invested his son and heir, born there, as Prince of Wales. Since then, Heirs to the English throne have been Princes of Wales. In 1969, the present Queen invested her son as Prince in the Castle, by then a ruin although its outer walls still stand. See image. BBC coverage of the 1969 Investiture plays on a loop in a restored chamber.
Macsen Wledig is the Ysan King Gratillonius' former commander, Magnus Maximus, who did rule part of the Western Roman Empire although not from Caernarfon.
Caernarfon Castle's "great towers of many colours" (Discover Caernarfon Castle, Cardiff, 2015, p. 3) sound like Ys.