The Western March (not to be confused with the Western Wild), was a county under the authority of the Kingdom of Narnia located between the Lamp-post and the Great Waterfall of western Narnia, east from the uninhabited nation known as "the Western Wild". The only known place of inhabitance there was the town of Chippingford.
King Edmund was Count of the Western March during the Golden Age.
In the Age of Winter, Tumnus mistakenly called this "country" "Spare Oom" and Digory Kirke's Wardrobe a "city" of "wardrobe" within it. In the Later Ages, for unknown reasons, outlaws from this region had waged two wars with the Narnian government, which implies that it's a breakaway state.