Although the geography in Gottfried's Tristan is inaccurate and semi-mythical, we can establish some approximate locations for the story's setting.
King Mark's kingdom is in Cornwall at the southwest tip of England.
Isolde comes from Ireland.
Tristan's home is more difficult to locate, but it is probably somewhere in the region around Brittany and Normandy on the French side of the English channel.
Major place names in the book:
Parmenie (Armenie, Ermenie)