A seaport in southern Britain.
According to the Alliterative Morte Arthure, Arthur kept a treasury in a vault at Wallingford, which included a magnificent sword named Clarent. The treasury were ransacked by Mordred during his insurrection. When Arthur learned of it, he knew that Guenevere had known its location.
Collier’s World Atlas and Gazetter lists no Wallingford in the British Isles, but it does list three in the States – in Connecticut, Pennsylvania, and Vermon (Rutland County).