The mention of Ephraim here is the topographical link that takes us from the southern setting of the Othniel and Ehud cycles to the northern setting. In terms of the schema of chapter 1 this is the first major episode which, with regard to both its setting and the Israelite tribes principally involved, falls within the sphere of the 'house of Joseph' (note the reference to Bethel in Judges 4:5).
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5 She used to sit under the palm of Deborah between Ramah and Bethel in the hill country of Ephraim, and the people of Israel came up to her for judgment.